For some reason Mint hates flatpaks by claiming they're giant, when they really aren't.
Most of the "size" comes from core dependencies, which do take up space. However, Flatpak comes as part of Mint, and the few dependecies not included do take some space. But definitely not what Mint Store reports.
I'd write it up to Synaptic package manager being a bit lazy and just downloading the entire list of dependencies and running with it, instead of checkig what the flatpak package manager will actually download.