Spike was I think about 4 when he seemed to be getting bigger, I think it may have to do with his fur starting to curl a bit lol.
Untill his 7th year he was always a bit underweight and when he was 5 we even had to get him to eat more to gain weight even though he already ate quite a lot but we were very active back then lol. After his 7th his arthritis became a problem though and slowly his activity has been decreasing, now he can barely walk for half an hour (that's just walking, no play of any kind). So since then he's also gained a bit of weight that we can't seem to lose even by decreasing his meals.