Transformation between aspect ratio
It looks to me like the vertical dimension is the one to use for scaling:

$$\frac{50\text{ px}}{720\text{ px}} \approx \frac{30\text{ px}}{480\text{ px}}$$

There's a slight error there, but it's only a few pixels. I'm not sure if the object sizes you mentioned were intended to be precise to the nearest pixel (30 and 50 are roundish numbers).

Typically when switching between aspect ratios, the vertical FOV would remain constant while the horizontal FOV widens or narrows to match the aspect ratio.