I'm not using packing cubes to fit more in, I'm using them to avoid my suitcase becoming a giant mess.
I'm not a naturally tidy person so they do help a lot with that.
Generally I go with 1x underwear 1x jumpers/vests/leggings/swimwear/cover-up (holiday dependent),1x dresses/outfits. That takes up 1 half of a Ryanair sized suitcase.
The other half doesn't have packing cubes and instead has shoes, goggles, jewellery, chargers, medication, toiletries, hairbrush. I do sometimes wonder if I should get some sort of cube system for that side as well but I haven't thus far.
My mum takes a dry bag for dirty washing, which is a level of organisation I haven't yet risen to.
If I am going away for just a night I can use my biggest packing cube in a normal sized rucksack to segregate it from all my day to day stuff/work stuff which also works quite well.
Mine were cheapo ones from Wilko's, not compression or anything.