No and No. Damisi is an average to large-sized male but still only quite young (13 years)- males tend to gain more weight as they get older so the biggest/heaviest males are likely to be from the older age classes.
Perhaps the largest Western Lowland Gorilla in a UK collection was Howletts 'Djoum'
But the largest overall was probably the Eastern Lowland male 'Mukisi' at Chester, as Eastern Lowlands are regarded as the largest of all the Gorilla species.
The prize for smallest male has to go to the unfortunate ex Rome/Bristol/London male 'Bobby' (originally Bongo 2nd) whose growth was evidently stunted from his early background (reared in a circus abroad)- he was definately the smallest male I've ever seen-anywhere, and his body was no larger than a big female(Bristol Romina) though he was heavier. Howlett's Koulliou is not large either, but he's normally proportioned.