I'm sorry to break your bubble (french expression) but you can't estimate the real memory that is taken by an application by checking the Task Manager... Windows is an intelligent operating system that often doesn't take the time to clean objects in memory as long as it's not necessary. As a result, an application can seem to take 20 times the memory it is really taking.
msgplus.exe doesn't do anything at all while it's running on your system, it's kept there for security reasons only. The process doesn't eats more than 128KB of real memory and has been specifically designed with memory constraints in mind (so that Plus! doesn't harm your system in any way and that you don't find excuses to remove it from the Windows startup and complain about other related problems
).
Messenger Plus! uses a noticeable amount of memory only in the Messenger process so you'll probably never be able to do a real estimate of its usage
. Don't worry, memory optimization is one of those things I'm spending time on, even if I'm not talking about it. For what I've seen, Plus! uses around 1MB of memory in the Messenger process, which, I think, is ok as Messenger itself generally uses 15MB or more.