What are you asking about exactly? Are you asking about the total download size, or the total size on disk once fully installed?
If your referring to the download size, the total download size will be larger then Steam's download size because the LR downloads each package file then unpacks them during the install. It's usually so fast that most of the time you don't even see it. This accounts for larger downloads then Steam because Steam uses a very different compression technology from LR. Once installed however, the size on disk of the installed version of X-Plane will be the same with the exception of an additional 4-16MB for Steam that's needed for Steam's licensing wrappers that get added to every Steam game you use.