Try using Autoscale Lite for the constraint and set its DominantAxis to Length
As stated in the reply below, using offset for position or size can cause this. You can use Autoscale Lite to convert position and size from offset to scale
It’s because you’re currently scaling it via the offset and not by scale.
If you scale it by offset, it pretty much scales based on the amount of pixels on the screen, with scale, it scales based on the percentage of the screen covered.
Change your offset values to scaling values and it’ll fix your problem.
(There are also plug-ins for this as well, as mentioned here;)
You may have to re-adjust your UI again due to the fact that off-set and scale are two completely different sizing positions, however plugins may also make it simple and avoid the process entirely.