Before Renaissance perspective was developed, artists used relative size to indicate things like relative importance (kings and Jesus were sometimes 20-50% larger than their followers), or timescale, or as you are suggesting, convenience to fit things in.
Basically, they didn't put much stock in making the scale realistic throughout the piece.
About the size, one elephant is smaller than a horse but has 4 small guys standing on it. Is it possible that it was a standard convention to get everyone in the drawing, instead of tying to make a photorealistic image.