with every brood that hatches out of a cell,the diameter decreases due to the coccon wall etc,when starting with 4.9 can your cells get too small after awhile?
No, not really.
When the combs get down into the natural range the bees themselves naturally build, then they do not try to make them smaller anymore and keep thinning the cell walls to the same thickness. 4.9mm top tolerance is in the center of this known range by the way and one reason it was choosen, besides the fact that secondary diseases come under control with this sizing by usage of field manipulation management only.
A good book to read containing this insite of keeping cells walls thinned,and therefore not letting them get smaller in size, is by EB Wedmore titled: A Manual of Beekeeping.
Dee A. Lusby
Although my oldest small cell comb is now 3 years old, I have also noticed that bees keep the cell walls almost as thin as they were drawn. And the bases are amazingly clean and clear as well.