Auto failed for vector size

I am trying use auto to infer the type.

for (auto i = remaining_group.size() - 1; i >= 0; --i) {
    std::cout << i;
}

I get very large number like 18446744073709534800 which is not expected. When I change auto to int it is the number I expected between 0 and 39.

Is there any reason auto will fail here?

remaining_group's type is a std::vector<lidar_point> and lidar_point is struct like:

struct LidarPoint {
  float x;
  float y;
  float z;
  uint8_t field1;
  double field2;
  uint8_t field3;
  uint16_t field4;
}