Why do Apps takes so much storage (3Gb) in the internal memory?

I have Sony Xperia M4 Aqua with 8Gb internal memory. The android system takes about 4Gb, and the applications take 3GB. But when I download an app, the first thing I do is to transfer it to the external SD. So why apps take so much storage from the internal memory? Maybe these apps are the original apps on the phone which cannot be transfered and deleted? Any ideas how I could free up more space? (I usually erase the cache, but it is usually only about 4-600Mb.) I would like to free up space from that 3Gb that I mentioned above.

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