We had one male who was 2" taller than the standard, and brindle. Two strikes against him in the AKC ring. Judges eyeball height & weight. I've never heard of a judge asking or measuring those two standards. But, supposedly, the judge is very well versed in the breed they're judging. It took a long time to get Sumo his CH. because he was so big.
And now, 10 years later, I see a lot of very large bullmastiff males in the ring. I'm pretty sure breeders will try to keep in standards, but on the top edge of height/weight.
ETA: My experience is in AKC. Some others will probably chime in shortly.