Because the width of size_t grows to 64 bits in X64, truncation may occur if you assign size_t values to a 32-bit integer in a 64-bit build. Though similar to pointer truncation, this problem is somewhat less dangerous because the high 32 bits are often zero. The following code results in a truncation warning in 64-bit compilation:
std::vector<int> myVec; long nVecSize = myVec.size(); // truncation warning
To guard against such issues, your code can check that the size_t value is less than 2GB before assigning it to a 32-bit variable.