DD and I managed to get all our gear into a Mini, but we had the full back seat as well as the boot for that.
Usually, we have a small estate with a half-size roof box (currently an Audi A3).
Tent (4 man Vango), I have a camp bed, DD has a sleeping mat. 2 small tables that fold away, 2 camping chairs that fold away, wind break.
10l water barrel. 1 ring gas stove and a small bucket style BBQ.
Most gear is in a 40l plastic crate, and a 22l plastic tub for kitchen equipment.
Add to that 2 decent sleeping bags, 2 pillows, a couple of spare blankets, picnic rugs, and some clothes, and some food. I tend to pack clothes in large Ikea Ziploc bags, either in types or as whole outfits, for ease of use, and these can then be squashed to get the air out like vacuum bags so take up less space.
I fill in nooks and crannies with small things, pillows and rugs have gone under people's feet (I sometimes carry Cub Scouts instead of DD), sleeping bags across knees ETC.
And avoid, as much as possible, things that are solid. So while I have a solid plastic tub for the kitchen stuff, I have a fabric coolbag that is sufficient for our needs and can be squished a bit. And I have left the larger tub behind on occasion, and only bring my bed when I have room for it (I have a thermarest that works ok if I need to condense gear, but I am still desperately seeking the nirvana of good sleep on camp, which is vital when minding Cubs especially!).