I think intuitively I expect things to behave "realistically", i.e. how hard something is to pick is related to it's bulkiness, heaviness, maybe squishiness. The intuition fails me however. Or the game fails my intuition I guess.

Usually surprisingly easy to pick up: Long, thin objects; weirdly shaped objects; boxy box type objects; very flat objects.

Harder to pick up than I would think: roundish objects, animate objects.

My theory on how it actually works: It's the volume of the collision box that matters. The collision box for boxes like milk cartons matches the model I imagine, for more rounded objects it's probably something like a bounding box, which has bigger volume than the model, hence why they seem harder to pick up. Very flat objects are basically boxes also, plus they seem bulky for their volume, which matters to my intuition but might not matter to the engine.

Long, thin objects like flowers, or weirdly shaped objects like carrots, have, I think, smaller collision boxes than they look. Possibly someone manually set these. Like the carrots might not have a collision box on the actual carrot bit. Or certain parts of a weirdly shaped object is not part of the hit box.

Animate objects like animals seem to have rather large collision boxes (maybe to make them easier to hit?). But it's very noticeable when you try to pick them up, and they do their little stun tumble. They always do that before the katamari even seems to hit them. For most objects, it either seems like the katamari picks them up exactly when it touches the thing, or sometimes you have to actually roll a bit through into things, indicates that hit box is smaller than the model would suggest.