Ok, it’s an issue of these limits not being strong enough to carry that heavy upper-body. They are tuned for a regular-sized human characters to remain semi-soft - like soft tissue - and to not look too robotic. As soon as a limb goes outside of its limit, there’s a spring created to try and pull it back. If that spring is too strong, it looks less like a human limit and more like a mechanical limit. It can be a bit of a balancing act finding a value whereby they remain soft but not too soft.
It’s possible these defaults need to be on the high-end rather than low-end, such that you instead find them too stiff and need to soften them. At the moment, it’s quite frequent that they are found to be too soft instead.
Here’s two alternative solutions you can try.
1. Increase Limit Stiffness Individually
2. Increase Limit Stiffness Globally
These will have the same effect, except one affects every limit in the solver.