Why is proportion so important in art?

When painting something realistically proportion is important, if the portions are wrong it will look less realistic.

Religion changed from primitive times through the renaissance period until today such that certain church conventions and rules were set about how the holy family could be portrayed. for instance: mary must always be in a blue mantle, jesus could only be in a white robe after the resurrection, jesus could no longer be seen pointing with his walking stick when performing a miracle, and so on.hope this