Small programs: possible pros: more cases to go around, better interpersonal relationships amongst residents and with staff. Possible cons: more call. Higher likelihood of not finding enough like-minded people (less diversity).
Large programs: pros: more people to share call, different personalities. Cons: interference by other learners
This does not include the obvious: research funding, academic caliber and overall volume. While these might be more prevelant in larger programs, I'd examine those separately.