if (_sizeInfo->dataSize.fetchAndAdd(amount) < 0)
_sizeInfo->dataSize.store(std::max(amount, int64_t(0))); 

Value is checked before adding, and clamping is done only if the previous value was negative, so the value can still be negative.

Additionally, similar to SERVER-63585 with fast count, data size can be incorrect after a rollback due to a race. We should allow dataSize to be negative internally and clamp only in the getter.