Good for you for recognizing a problem! Yeah, being half out of the seat definitely isn't safe.
Luckily, many of the car seat manufacturers have started making higher-weight harnessing seats. Previously, there were only a handful of seats that harnessed above 40 lbs, and they were pricey. A good thing to look for is not only the weight that the seat can harness but also the height of the top harness slot. Most of the time, children outgrow their car seat by height rather than by weight. Once your child's shoulder's are above the top harness slots, they've outgrown their car seat.The Graco Nautilus is a great seat that harnesses to 65 lbs then becomes a good booster. Also, the Evenflo Triumph Advance harnesses to 55 lbs but has a nice high shell, so most kids can make it to the full weight limit. (make sure you look for that "advance" part, as the Evenflo Triumph only harnesses to 40 lbs and has a much shorter shell). The big Britax seats (the Boulevard, Marathon, and Decathlon) are also great seats because they have high weight and height limits and are very easy to install and use. Of course, the best car seat is one that fits your child, fits your car, and can be used correctly every time.
As for strollers, most have weight limits of 40 lbs, although I know the Maclaren strollers have higher limits and fit taller kids as well.