Actually the more heavier and muscle mass you build the more stronger you should be in the chins or any other exercise. That's the whole idea behind serious training . So approaching chinning with the 5X5 system, for example, anyone should be able, after awhile, to be using some pretty impressive weight in chinning.
Just because anyone gains that muscular weight does not mean that their strength in the chin will stay the same or be even less because of increased bwt. Seen big men do sets of 2's or 3's in the chin with quite a few plates. Trouble is, most BB'er's think higher reps should always be done with chinning. Not true, treat it like a power movement and see what happens.
Some men have a natural stronger ratio pull than to their push. These are the guy's who can chin, and reap the benefits of this mass builder, advancing up to using heavier weights strapped on. Reverse is true of a natural stronger push. These guy's seem to have not all that trouble using 400 and above after awhile. Guy's who want to up their benching may do well by adding heavy chins to their programs.
Gironda made the wide grip chin, touching the upper chest to the bar each rep and hold for a second, popular. Hits the full contraction of the lat's and other back muscle groups. Most guy's pull to about eye to chin level. At times that may include half reps. Whatever method work for you, use it to your advantage. (hint: partial reps and heavy negatives work very well when wanting to increase size and power in any form of chin's).
With pulldowns, the weight of the hips/butt/ legs are taken out of the equation. With full chins, they are not. You are lifting more full bwt. Which makes it a more demanding and productive exercise for most. Old BB'ing theory that a body lifted lifted in space (like chinning or dips) are more effected for mass building. Interesting anyway. Good Luck.