...Not so keen on the 20X30cm box as the "lie" either. Seems like it makes it harder to discern if one is on the lie even more. Though it does open up a market place for lie markers...a 20X30cm piece of cloth or plastic that one can place on the ground behind the marker disc (or in lieu of). If you step on the lie marker, you're legal.
So the length of the lie stayed the same, but the width has been extended to just under 8". The idea was likely to use roughly the width of the disc as a guide, and in practice I'm pretty sure that's what players will go by. And as far as placing something behind the mini, that would lead to the potential of the mini getting bumped and moved during the plant/pivot.