or there is security from that?
This site is intended to be one question per post and your questions don't really make sense. But maybe if I answer your first question you can use that knowledge to improve your further questions and ask those separately.
Blocks do NOT have empty space. 60% full means the size is 60% of the limit, but of course only that 60% is sent over the network and stored.
If the limit gets raised to 20MB it does NOT mean all blocks will be 20MB. The size will still be exactly the same as before until demand starts growing and there are actually more transactions to be put into those blocks.
Smart people has done it already and made an interesting stress-test that You, my dear friend, can read about right here - this is Part 5 that have links to other parts as well. Probably you want to start from the beginning then have a good read!