Your surface-area and temperature dissipation theory is quite good, I believe
What I was getting at is that the 5v rail of a computer PSU is not known for its stability. It only takes a small burp to jump above 6.3v. Most circuits inside a computer are protected from such overvoltage, though a cap stands no chance and is permanently damaged each time. I must say I've never had issues with ESR. The capacitor I used to replace my bad caps was just a random capacitor from my electronics box. It wasn't really "low ESR" or any of that "high quality" stuff. There's much debate about "low ESR" in the audiophile fora elsewhere on the internet.
Update: for whoever is interested, this is a picture of my old geforce 7600 with three (!) popped 1000uf 6.3v caps (2 at the top right, 1 at the bottom left)
I tried desoldering them with a desoldering tool. As I expected, lead-free Pb is very difficult to deal with. I eventually just pulled them out destructively with pliers
The three broken ones were replaced by three 1000uF 25v from my electronics box. They aren't as "good looking" as the solid caps that were on there (if they even were solid caps, who knows), but these ones have never failed me. They're not "LOW ESR", but they work and my old computer can continue doing pointless things all day!