For me, it is easy to understand why, it has to do with shipping code sooner to your customers. It takes a lot of resources and time to optimize your code and the longer your customers have to wait, the more money you lose. In many cases, the extra size is due to frameworks added to the programs to speed up the development.
Frameworks tend to be heavy in size because it needs to accommodate various tasks as well as many platforms it can support. It's not easy to make them modular, so you can pick what you want and leave the rest out to shrink down the size.
For customers, would you rather wait a few days to get a certain feature that works out good enough in a few days or would you rather wait a few months for a feature that works great?
The competition is intense, wait too long to ship a feature and you lose to competitors that managed to get it out sooner than you. So, it's tough to balance each feature and tough to say no to customers, so that you could stay focused and lean.