In Addictive Science when a transformation changes someone's mass a "random" nearby person (usually Lukas or Mark) turns into something with an equivalent mass change.
In Cat Nine, Myan can transform into smaller or bigger animals (or a cat girl) without any problem because of her magic collar.
Averted in Digger: After turning into a huge monster, Shadowchild points out that in that shape it is stretched out thin and can't do much besides looking scary, and this is with Shadowchild being not exactly solid, but presumably magical, to begin with.
Averted in Drowtales: Ariel uses whatever she has around to shapeshift into something bigger (such as creating wings), usually her hair and clothes but also a small dragon golem that was created with that ability in mind.
Grace could originally change her density, but not her mass, which resulted in her squirrelly form being the same weight as her human form despite being a lot smaller. Then the TF Gun was introduced, which didn't obey the conservation of energy or mass at all, and Grace getting hit with it permanently "fixed" her shapeshifting to break those same laws. Somewhat justified, in that despite the It's Magic! explanation, there are still rules to it, and Tedd creates a whole new field of study out of figuring out the laws and limitations of that magic.
Elliot ends upwondering how he has his car keys after doing a series of costume-changing transformations (none of which have pockets). Apparently, if he has anything in his pockets when he shapeshifts, it temporarily merges into Elliot. Such as his smartphone.
In Girl Genius, when Queen Alba shrinks down to (slightly over) normal human size too quickly, she overheats, suggesting she converts mass into energy. This doesn't explain where it comes from when she reverses the process, of course.
Snookums the Tentacle Bunny in The Inexplicable Adventures of Bob! was originally a Kaiju-sized giant monster, but he got shrunk to the size of a basketball and is now kept by Molly as a pet. He still retains all his original mass, though, so the earth shakes every time he hops around like a bunny rabbit.
The Ambis in Jix all can transform into ugly beast versions of themselves and the more skilled can grow several times larger, such as Kelelder the Planet Thief, his daughter, and Maricax (a bounty hunter). It's never fully explained where the mass comes from or goes when they revert back to their normal size, though the creator hints at the Animorphs explanation where it's pulled from a pocket universe.
L's Empire: Snowball can multiply his size but can't change his mass. This results in a large —but ultimately very light— penguin.
In Nodwick #4, the protagonist is forced to drink a magic potion of giant size to fool an invading army of orcs. As it turns out, the potion doesn't increase his mass, only his size, and his increased surface area leads him to be blown away by the ambient breeze.
Yeagar: Does anyone know why our fifty-foot henchman was just whisked into the sky by a thirty mile-an-hour breeze? Artax: I might have an answer for you... Artax: According to the fine print for "Plan #1", the growth potion increases volume, but not mass.
The Non-Adventures of Wonderella: "My CINNAMONS Exactly" parodies this. Cinnaman, a shapeshifting villain made of cinnamon, can't transform into a larger horse because he doesn't possess enough mass. Wonderita asks why he doesn't just buy more cinnamon at the supermarket. Cut to Cinnaman, now 100 stories tall, rampaging through town.
An Oglaf monster can't violate conservation of mass, but people aren't fooled by a giant abandoned baby that's the size of 300 babies. So it turns into a pile of 300 babies.
The Order of the Stick does a Deconstructive Parody - The mass a Hydra gathers from thin air to add to its new heads is turned into a source of free food once the monster is captured. And those villagers never went hungry again.
In Peter Is the Wolf weres gain some mass when they shift into beast form, but the actual amount varies from "runts" like Peter who only gain a couple inches of height (and several more inches elsewhere), to Sarah who goes from five foot nothing and skinny to over eight feet tall and stacked. One character claims this is evidence that lycanthropy is supernatural (as there's some debate in-universe).
In Skin Deep some of the monsters are significantly bigger in their natural form compared to their disguised human form. This relies on the magic of medallions, which (currently) no-one understands the workings of.
Kieri from Slightly Damned switches between being an angel and a snow bunny without much hassle. Considering her clothing pops in and out of existence as well, it's fairly safe to assume that it's guardian magic.
In Star Trip, the shapeshifting alien Khut is capable of changing their mass with their form, varying anywhere between a small animal a human can hold in their arms up to a form outclassing gigantic combat robots. It's noted that this should be physically impossible, as this is a science fiction comic where conservation of mass is normally in effect, but no one knows how Khut does it. There's also Khudran, a self-aware fragment of Khut who generally takes even larger forms.
Tales of the Questor specifically averts this with bogeymen. No matter what form they take, they retain their original (minuscule) strength and mass, to the point that a bogey taking on a really big form is essentially a cloud of blue gas. Played ruthlessly straight with spriggans... they hit really hard.
wondering how he has his car keys after doing a series of costume-changing transformations (none of which have pockets). Apparently, if he has anything in his pockets when he shapeshifts, it temporarily merges into Elliot. Such as his smartphone.