Из обзора олинова блина с дпревью:
Other brands also have their own small primes such as 28mm F2.8s or 35mm F2s, but a Canon EOS 1000D fitted with a 28mm F2.8 lens will be fully 25mm/1" (i.e. 33%) thicker than an E-420 with a 25mm F2.8; this combination really is about as small as it gets for DSLRs.
http://www.dpreview.com/lensreviews/olympus_25_2p8_o20/