Basically, all bikes 50cm and shorter are the same. The top tube can't be any shorter or you'll have the dreaded toe overlap. Toe overlap is not a big deal, but some people fear it so bike manufacturers do all they can to avoid it. The head tube can't be any shorter, you need enough to connect the down tube and the top tube, with enough room for a quill stem if that is what is used.
To shorten the top tube and alleviate the toe overlap, manufacturers will make steep seat tube angles and lax head tube angles. But this will only change the top tube by about 1.5 cm at most. Another option is to raise the bottom bracket, the top tube can be shorter, but the stand over is higher. One other option to shorten the top tube without having toe overlap is to use smaller wheels, but manufacturers don't do this often because they believe shorter people won't like this.

Newer bikes have compact frames, so instead of a level top tube, it angles downward. But again, the head tube will still be at the same height as a traditional frame. This can give you the difference in seat tube angle with just a bit smaller stand over height.

I would have to look at the bikes, but most likely, the 48 and 42 are probably almost identical.