Infa Secure Vario is one of the narrowest boosters. Maxi Cosi Goliath is my pick for Convertible booster. It has also been released as a couple other names.

(from my excel file on car seat dimensions)
Infa Secure Vario Booster 4 y - 8 y 430mm width
Safe-n-Sound Maxi Rider Easy Adjust Convertible Booster 6 m - 8 y 7kg 460mm width
Maxi Cosi Goliath Convertible Booster 6 m - 8 y 5.1kg 450mm width
Are those widths for the top of the seat (where the wings are) - or the base. I know the Comfi Caprice from Infasecure is 50cm at the top - which is the same as my Ezy Love one. Infasecure say the Vario Booster is 400mm at the top - so I like the sound of that one for the Booster.

DS although only nearly 3 is very close to the convert to booster mark on the Ezy Love Combo - so maybe he should just have that one, and then can go into a thinner just Booster earlier - he is as big as some 4 year olds I know.

Target had some Mothers Choice Seats in which looked really narrow at the top.

Researching car seats proved disastrous today, I went down to measure ours and locked myself out with only a tape measure and the car keys, so I had to drive to DH's work and get his keys off him.