Low water pressure in bathroom in the first floor
Increasing the pipe size from 25mm to 50mm will definitely reduce the pressure loss along the pipeline, making more pressure available; however, 25mm is already a pretty good size for a residential water pipe and the pressure loss may already be very small.  So the difference may not be noticeable.  Unfortunately, the problem is likely just the lack of vertical head from the tank to the bathroom.
In some places they make tanks with a bladder or spring inside which are pressurized as they are filled to provide more pressure than gravity alone can.