Especially if you compile it yourself, it can be very light in memory usage.

Many working examples to study and learn. You can easily experiment modifying an example code to grok in full how the engine works. In the beginning of your learning curve, the example code will offer you many useful code snippets.

If you want to start game developing with C++, then Irrlicht is a good candidate since it removes most of the complications in game making in C++. It will let you obtain experience in programming and games at the same time.

If you are considering writing your own engine with openGL, you might want to consider Irrlicht instead. It makes many features of any engine worth its salt easy, including events, serialization, nodes, animators, materials, logging, and animation. Bring your own sound and networking.