I honestly think it has a lot more to do with personalities of the kids and sometimes gender as well. One reason why my kids are 2 yrs apart is b/c my Dh and SIL are 7 yrs apart and she always felt more like an aunt to him than an older sister. Dh didn't want our kids to have that kind of relationship. I tried to relay that it wasn't so much the age gap, but I'm not sure he was convinced at the time. It didn't matter too much to me so we had them fairly close (he was deployed or they may have been closer in age).
Now what's weird is that we ended up having our kids at around the same time as SIL (they're all within 10 mnths at most of each other), so it's made Dh/SIL's relationship a little easier as adults, b/c they can relate to each other as parents of similar aged kids. They didn't have anything in common as kids to kind of bridge their age gap.
I also think a parent with a big kid-gap has to be careful not to put too much responsibility on the older sibling to be another parent or de facto babysitter. I see this a lot in a large family in our neighborhood and the older sibling is extremely resentful of having to miss out on doing things with kids her age because she has to tag along with her 8 yr old sister's escapades. She also can't do any after-school clubs at school as she needs to watch her younger siblings at home. This is usually the exception instead of the rule (I don't think by any stretch that *all* large families are like this) but it's something to watch out for. I know for *me*, the temptation to lean too much on a responsible older kid would be hard to resist vs. paying for a sitter. But I'm cheap.