We currently have a Britax Marathon convertible seat for my 3.5 year old who is 42 lbs and 40 inches. He's in the height and weight range for the seat, but he's kind of built like a tiny linebacker and we've been noticing how squished he seems lately. Not to mention that it doesn't seem like the straps actually extend any further than they're already out.

So I'm trying to do some research on 5 point harness booster seats to see if there's much of a difference in size than a convertible seat. Has anyone with a preschooler already moved out of the convertible? Recommendations for boosters? Obviously safety is our #1 concern.