Since the film opens with Jacquimo settling in to tell us the story, the theory goes that he basically tweaked certain parts of it by introducing himself earlier than when the swallow normally appears, which is in the mole's tunnels. Instead, he instigates Thumbelina's rescue from the toads and inspires her not to give up and to find her way home to her mother, turning her off from the original narrative where she becomes a survivalist who never sees her mother again. He also has her meet the fairy prince earlier in the story so that he can spend the majority of it looking for him, because that makes for a better, more dramatic and emotional story than one where Thumbelina doesn't meet him until the very end, when the action has wrapped up.
And in large part, this is intended to close the plot hole of why Jacquimo couldn't just fly Thumbelina home himself. In the actual sequence of events, he didn't meet her until much later and flew her to safety the first chance he got, whereas the version being narrated by the film is just him trying to tell a stronger story, not one that makes complete logical sense, hence there were some things that he forgot to narrate in (like why Thumbelina's mother mentioning wanting a little child) which creates more plot holes.
