The breed standard also states that the Boxer is a medium-sized breed - so even though there are no height disqualifications, a small-sized Boxer would not be a medium-sized breed! (Also, the standard does note males should be preferably not under the minimum.)
Also, I think it would be extremely hard to breed miniatures without pulling in another breed. I know from personal experience that breeding small to small does not equal small - my female is 21 inches - small but within standard - and *every* puppy she's had is taller than her. The studs were 25 and 24 inches, respectively - again within standard - the males pups that I've seen recently are all 24-25 inches, the female is 22 1/2.
Some people may think you have a miniature Boxer because "we had a Boxer when I was a kid, but it was much bigger than yours." In reality, the *person* is much bigger now so the dog looks much smaller - just like when you go back into your elementary school as an adult and the drinking fountains that you could barely reach then only come up to your knees now
