What you’re describing is due to using sprites with different hitboxes per animation. Because your hitbox changes, so does the available space for collision detection.
You need to separate your art assets (sprite with animations) entirely from your platformer character object.
You can do this by having a normal square (that matches your “normal” character size) that doesn’t change size at all depending on animation. You add the platformer character behavior to it. Then you just place another object with your character sprite, and use events to keep it’s position linked with the actual platformer object.
If you look at the built in platformer example, it is set up this wya.