Imagine you have two boxes, each with a photon of equal frequency inside the boxes. The boxes, however, have different side lengths (yet, the boxes on their own are equal in mass). Wavelength of photon << Length of Box A < Length of Box B I've seen that a photon increases the mass of the boxes. Do these boxes with the photon inside have the same mass? I was thinking that if you make the box very very large, then photon bounces become less frequent, almost to the point where there will be long periods where the photon has not bounced inside. This would make it look like the larger box has less of 'something'.