@mubertmumphrey: If it's using CFF (cubic) curves instead of TT (quadratic) curves, they're usually a bit smaller. Also, no TrueType bytecode hinting instructions, which can take up a shitload of space

. Also depends on how much your software compress the font file itself

. Bottom line, just saying 'OpenType' doesn't really tell you much about how the font is really outlined

.
The foundry will usually give a more accurate description, like OpenType CFF, OpenType PS/Type2, or OpenType TrueType, so that's not really a problem when you're actually browsing/buying fonts, but when you stumble upon a file on your hard-drive, it can be a pain to tell the difference unless you know what & how to look for

.