Allow Fluid size as secondary size larger than primary size

### Description

Not sure if this should be a new feature request or bug request (or if I'm missing something), but currently when I set fluid as secondary size, the size is limited by the primary size.

I have something like this:
``

When the Fluid size is returned it takes the width of the primary size (300px) instead of the fluid's width. This doesn't happen with the height, which is correctly rendered when fluid is returned.

### Alternatives Considered

ignore/override the width of primary size when the fluid is rendered

### Additional Context

Please let me know if I'm missing something or if there is an alternative to this, I know you have a lot of work!

Start by reproducing the reported amp-ad markup with width="300", height="250", and data-multi-size="fluid", then trace the code handling fluid secondary sizes. Done means a returned fluid size can use its own width instead of being limited to the primary 300px width, while height behavior remains correct.

Written by the indexing model from the issue text.