Well I took a look around;
Yanmar 3ym30 (30HP)($2.3k without saildrive)
1.5gal/hr full out
So size is essentially a wash. Generator is 100lbs heavier but also uses quite a bit less fuel(~.6gal/hr). Price tag hurts at over three times the price, but you are also comparing a 1800 vs 3600 rpm motor(lives twice as long).
The THOOSA 9000(9kw drive) is also $8k,requires 48V, and weighs 50lbs.