Big textures take a lot of space, and yes its very easy to go over 100mb with unreal.
There is more, it eats like 30-40mb for breakfast. Most is engine code but there is some wasted data (textures that are used in editor only).
Unreal is next gen engine, it is meant for top line products that are available now (like nvidia shield, or iphone 6)
You do not develop 3d game overnight, you will do stuff for few months to year or two, in this time 100mb should be not that big deal.
It is already peanuts if you are on wifi. Next gen phones with 64gb memory will be fine.
And it is not a bug, rather feature, or lack of optimization. However i think unreal never will get smaller than 30mb that is used for engine.
Hopefully they get rid of that extra wasted data.