Well, you need to first understand how BMI works. BMI is the relationship of proportions between your weight (kg) and height (m). To calculate BMI, you do (w/h)/h where w=weight (kg) and h=height (m). This formula simplified is the same as (w/h^2). So, its now just to calculate.
Mankey has h=0.51m ~ 1'8" and w=28kg. 28kg ~ 61.7lbs. So...
(28/0.51^2) ~ 107.7
Snorlax has a h=2.11m ~ 6'11" and w=460kg ~ 1014.1lbs. So...
(460/2.11^2) ~ 103.3
I think the misconception you have is based off of weight, which is why you think Snorlax should have the greater BMI. But if you look at Mankey, it weights about 62lbs, but it's shorter than 2 feet. That is very heavy if you were comparing to a same-size human being. If you look at Snorlax, although it weighs 1000+lbs, it is also about 7 feet tall. Very few human being reach about this height, so a large weight for such a Pokemon is not that peculiar. If you think of it in proportions, it would make sense for Mankey to have a higher BMI.
Hope I helped! :)