Milo's weight was less on Mars than Earth, which would be correct. However, when Gribble and Ki are on Earth, their weight should be higher than on Mars - by a factor of approximately three. This would have made it impossible for them to walk or really move around much at all.
Milo's Mom told him that cats are not supposed to eat vegetables. Cats can actually eat some vegetables like cooked/steamed broccoli and some others.