I think its just popularity. The 140 seems to have a big following. Its not has heavy as the other three as well.
The 145 has the largest chest at 80cm but has smallest hips at 75cm
The 153 has a slightly smaller chest at 78cm but has 82cm hips
The 156 has the smallest 73cm chest but larger hips at 79cm
As you can see they all look fantastic so you have to ask yourself ''Do I want big boobs or a big bum?'' Personally I would go for taller rather than worry about cup size but the best thing to do is choose the one that floats your boat best.
PS I chose a YL 155 custom and I'm a very happy bunny.