Yes, though not too small, 14kilo. I have a cover which has straps that go over the rear headrests and covers the full seat. Her harness clips into a seatbelt buckle through a slot in the cover. The only hassle is if she moves to the centre seat she can set off the seatbelt unfastened warning beeps.
We have a Springer Spaniel who occasionally comes in my M3. He is happy enough being fixed in via seat belt connection on the back seat. I do find the car is easier to clean than others after our dog has been in. Also found dog mode really handy over summer. Still no replacement for a hatchback car though in my opinion.