Please help me understand wheel size/tyre profile

Hi guys



Low end cars here but please let me understand some stuff.



My car (318ti) has 17" wheels and tyres that seem to measure 11cm thick from wheel rim to outer edge of tyre. The ride is so rough that even driving over a feather feels like someone is kicking me in the backside. I took it to a mechanic who told me the shocks and springs were fine.



My sister has almost the same car (316ti) with 15" wheels and tyres that seem to measure 16cm thick. Her car glides and is such an easy ride. You barely hear the road or feel much.



Both cars are about the same age, mine has about 35k more on the clock that hers but the bodies are the same I believe.



So can you guys explain to me what the difference in ride is between her smaller wheels and bigger tyres vs my bigger wheels and lower profile tyres?



I'd like to keep the aesthetic look of my 17" wheels and lower tyres but the ride is just horrible and am wondering if there is a way to make the ride feel more like her car? Can other tyres help? I should point out my tyres are real cheaper chinese tyres labelled "Ironman".



My apologies if this is a bit of a lame query.



Thanks