In response to frame sizing questions I've often seen the advice "slightly too small is better than slightly too large". (I've seen the opposite as well, but much less frequently). This advice rarely comes with any evidence or anecdotes as to why a slightly-too-large frame is worse.

It seems like, in either case, adjustments to saddle position and stem angle/reach could produce a pretty much identical fit. Right? Assuming the exact same triangle between handlebars/bottom bracket/saddle can be achieved by two frames, is there really any reason to prefer the smaller one?

I guess the smaller frame would be slightly lighter, easier to mount/dismount, less chance of smashing one's testicles on the top tube... I guess steering would also be different (better?) with a longer stem on a smaller frame... but I don't know if these issues are what people have in mind when giving advice to err on the small side.

So, for anyone who has given said advice, I'm curious: what is the reasoning? what are your experiences? How does frame size contribute to ride quality/comfort?

Thanks. (I know this question isn't fixie/single-speed specific, but I've been lurking in this forum for a while so it seemed an appropriate place for my first post.)