There are many reasons why apps are so big, but I think you are right about a major part of the problem. Development environments these days make it very easy to add in third party libraries for very little effort.

At a previous job we had a monolithic java server that ended up at over 350Mb of compiled code simply because each development team had imported whatever libraries they thought they needed. In some cases, 2 or 3 versions of the same library were included.