Texture file size variation problem!

Hello all,
I have run into a problem regarding texture file size variation between the external texturing software, the sizemap in the engine and the file saved as UASSET format in the project directory.
I am attaching three screenshots:
Why is there such variation of file sizes and how does it affect the project? How does it affect the build?
Lastly, is there any way to optimize the file sizes to have them uniform across all locations?
If there is anything else I am missing, please feel free to point it out.
All help is appreciated,