Um, er, ah, Ian, within a prescription every dimension -- radii, elements' thickness, spacing between elements -- scales with focal length. So, for example, the front surface of a 150/6.3 Tessar is relatively flatter than than the front surface of an 85/6.3 Tessar.
Paul, be more specific. Which lenses are you comparing? There are many many lenses for every format and some are quite, um, bulbous. Others with the the same or greater coverage aren't.
For example, Ian, I have an extremely bulbous 260/10 Nikkor-Q -- this lens, in fact: http://homepage2.nifty.com/akiyanroo...po/pro260.html ; Akiyan's image circles are at 1:1 -- that covers 75 degrees at infinity. I also have a 38/4.5 Biogon that covers 90 degrees. Its front surface is relatively flat.