Most kids outgrow seats by height before weight -- higher weight seats specifically targeted for obese children usually also have higher top harness slots so tall/thin kids can stay safer harness, too....
MaxiCosi is the name of the company which has 3 different types of carseats in the U.S. This particular child already meets the maximum weight limit of the harness & would have to use their
booster. This is fine so long as the child will stay properly positioned as still as a crash test dummy for the entire length of every ride without ever slumping or leaning or pulling on the shoulder belt. Many kids are simply not mature enough for a
booster until outgrowing higher weight harness seats.
Carseat laws varry by state, but the laws of physics don't care where you live. The longer kids stay
rear-facing within the limits, the safer the are. The longer they stay harnessed within the limits, the safer they are. The longer they stay in a
booster within the limits, the safer they are. The longer they stay in the back, the safer they are. Each "graduation" in seat type is a DEMOTION in safety.
Are you asking for your friend's child or for your own? Are you pregnant? What is your child's age, weight, height? What is your vehicle year, make, model? What is your budget?