I have a breed project (don't we all???) in the very early stages, and put a dual-purpose Roo is mixed heritage over all my birds, including my Brahma. As others have mentioned, the size gets there **eventually** but I can't make it cost effective. They don't bulk up quickly, and a single cross with a fast growth bird doesn't make enough of a difference to really change the calculus.
I'm on F2 now, putting my second largest Roo (tl;dr:, he was actually fastest to sexual maturity, has the best coloration, and is close in size to his larger sibling - another consideration in my project) back over the original hens and a select few of his generation to see what comes of it, but at this point, where the Brahma mammas are involved, I'm more stubborn than cautiously optimistic.
My Brahma were also by way of TSC, and are larger at age 1 year, but not the monsters I've seen other posters take pictures with. They free range, with a once a day feeding (evenings) to ensure they sleep with full crops.