Probably because excess calories from a fat source are the most likely to be stored in adipose tissue. Whereas carbs are middle of the road and protein the least.Originally Posted by max silver
As an ectomorph myself I find that I just build muscle very slowly. If I use a daily calorie surplus that typical for endo's and meso's I add too much fat. IMO a more conservative surplus and a longer bulking phase is probably the better approach for ecto's.