Actually, the real reason is so that a judge doesn't need an inordinate amount of time to check in your deck. The deck sizes represent the largest deck allowed for the number of allowed unique enhancements (for example, in a Type 1 deck, you can only have 3 copies of Authority of Christ, so 150 is the largest needed). 154 and 252 are the lost-soul cutoffs after 150 and 250 cards.
Kevin Shride
