turtle / flat turnovers -- are there particular ones that never work on higher weightclass?

that has been my experience, that some turtle attacks are pointless to attempt if the player is much bigger than you.  Would you agree ?   If you agree, please describe more examples. Or is it just my lack of skill?

For example, I believe i have tried before: straddling turtle, facing the same way as him.  Grab deep underhooks, high up on his lapel.  Do a forward roll, and he should be forced to follow you over.  This can work well for similar weights, but heavier/bigger people can ignore/resist your attempt to move them.

Are you rolling straight forward, or more to the side? (ie over uke's shoulder towards his head?)
There can be physical limitations to any technique, but the better the execution, the further those physical limitations will be pushed away.
Conversely, the greater the skill the of the person defending, the earlier those limitations will enter into play.
Everything is relative. Size, strength, speed and skill all interact and interplay between tori and uke to determine what will be effective. It's futile to try to establish fixed rules. Better to try to train and problem solve your way around the obstacles you encounter.

Even in something so simple technique matters.

When I was a teen competitor many years ago, they didn't have 15 percent rule in the weight divisions. I was super skinny--6 feet tall and 115 pounds. I used to have to fight guys that were 160 to 175 pounds in tournaments. I just developed a strategy to use technique and throw for ippon. If I got a koka or a yuko, I'd scramble back onto my feet and get back up to fight. I didn't stand a chance on the ground with guys that much bigger and stronger. I really didn't develop a decent ground game until I grew up. It's much easier to win against a big guy on your feet.

GregW wrote:It's much easier to win against a big guy on your feet.

I think that's a matter of personal aptitude, because for myself I've always found it to be exactly the opposite.

See I compete in masters and there are some big guys not everything works on everyone.