Maybe look up some vids on Youtube that shows the borders as it changed. It was already huge even before Christianity was legalized. If there were military campaigns during the time it was finally Christian-majority (which still took centuries after Constantine), it were continuation of past conflicts with surrounding polities, like Persia for example.

Compare this with Islam. All those conquest was definitely for religion. Unlike Christianity which mainly spread among the lower-class peacefully (heck, they were even fed to lions or impaled and burned on stakes), Islam was spread by the sword, with the army led by no other than Muhammad himself.

And oh, Catholicism came later during the Great Schism.