I was thinking if he got destroyed he got destroyed. Doesn't make sense for it to matter how powerful the blast was, since the amount of tissue he's rebuilding is the same, but GSM123 makes a good point about head regeneration probs taking more energi.
I was also thinking, after checking the anime and manga, I guess it's possible he quickly tried to dampen the KHH's damage by sending out some ki to protect his body. Watching/reading the KHH, it looks like the part of Cell destroying doesn't cover 100% of the area of where Goku's KHH probably went. So he likely used some ki dampening it. With the FF it seems like he didn't use ki to dampen it, as it just hit a smaller part of him and totally destroyed that whole area.