Giant isn't like dwarfing in that it isn't controlled by one specific gene. It's really more like careful selection of genetic modifiers over time, and such, you won't get some giant and some non-giant offspring, like you do with dwarves.
Crosses will not be as large as purebred, you can still carefully select for the largest offspring to maintain size, but the best advice is to breed only what you want more of.
If you want giants in the future, I strongly recommend only breeding giants to giants. It takes a long time to select for size like that, and it will take a long time to "breed out" smaller modifiers once you add it into a line.
