All boxers come from German Lines.....both come in all shapes and sizes. But I have noticed that the boxer's bred in this country are bred for their size and stature, not necessarily by the standards of the breed. Plus, breeders here are much more apt to breed for profit and NOT for the betterment of the breed and keeping the lines pure.
I know here in Amish Country, boxers are often NOT standard, as they mix-breed and cross-breed between their families dogs. So, it may say AKC registered, but there is no quality control per se in the breed lines. They also lie on their AKC registration because they know those 3 little letters means more profit! Often, the sire or dam listed on their lineage is not correct. They will breed a non-standard with another non-standard and end up with a boxer that doesn't really look full-blooded, or boxerish at all. Americans seem to breed willy-nilly and often breed boxer's that NEVER should be bred at all. If we think they are cute, then "let's breed it"!
The best way to ensure you have a standard, full-blooded, traditional boxer is to deal with a reputible breeder with a long line of quality, if not, show dogs in the lineage. You probably won't find them advertising in local papers, on craigslist or any other advert publication other than breed mags or breeder literature. They don't have to advertise, their reputation upholds a long list of waiting suiters.
If she's 50 lbs at 8 months I HIGHLY doubt she will get to be 90-110 lbs. My female Caney weighed 55 lbs at 9 months old (when she was spayed) and she's now ~63 lbs and almost 4 years old.