Expand Dong - I don't know if this kind of stuff is still going on but I just felt like making this

Actually, it's because there isn't any model sheets. Thus, the animators have no reference. Saying that it's because it's hand-drawn is a insult to every hand-drawn show that doesn't have this problem.

Meanwhile, other cartoons even decades old ones made with the hand-drawn method manage to stay consistent with the height and stuff.

Like ComputerPlayer said, a model sheet is a must for consistency.