There's probably not as many small engine mechanics out there so that'll accentuate a smaller number of injuries and deaths.
but also, as a small engine mechanic you're working with an explosive device. It's controlled explosion, but it's been designed to be compact and lightweight while still providing a decent power output. I imagine that if anything goes wrong, that thing turns into an IED pretty quick.
Not to mention that some small engines use fancy fuel that can fuck you up pretty quick, and all engines are prone to catching fire.