The OS doesn't necessarily have to be super-tight on the thighs. The legs are probably the weak point of any one-size diaper, since it can't be adjusted at both the waist and thighs as can something like the Sandys. But there are other advantages, as you note, and generally it's not an issue. If you are using the diaper with a good cover, like the Airflow or Rikki, you'll see that you won't have to worry about poopsplosions. Although it seems like the diaper and cover would have to be pretty darn tight around their legs, I've found that that really isn't the case.
I've had a few leg-leak issues (just onto the cover, not onto clothes) with my 26-lb boy who has chicken thighs and loose BMs, but in general have been very happy with the OS, and especially happy with them when I had two in diapers at the same time and could use the same diaper. My DS#2 has always had skinny legs and I didn't have any problems until his legs got even skinner _and_ he was racing around after stealth pooping.
There are pros and cons of the OS and the Sandys, but if you think your babe's proportions might benefit from being able to adjust the thighs, get a trial of the S Sandys, too (or see if there are any used ones on the buy/sell forum).