ATMs are but one measure of an airport's size / importance.
By measure of cargo EMA is already the biggest airport in the Midlands - actually one of the biggest outside LHR. I think I'm right in saying that if you take account only of pure freighter tonnage EMA is probably the biggest in UK.
By passenger ATM's EMA has a mountain to climb. However I think if you compare total ATMs BHX vs EMA it's going to be closer, though I still cannot see the day when EMA gets near the BHX total figure.
By the most used measure (passengers - or bums on seats) then BHX is still way ahead, and will continue to be so.
I think that as with most figures, there's lies, darned lies, and statistics and the "spinmeisters" with continue to quote the ones most convenient to their cause, whatever the truth might be.