If one circular object A (such as a hair rubber band) has the same radius at rest as another B, but stretches to a larger radius than B is able to, is there a way to say "A is xxxxxer than B"?
Typically, round objects are sized when at rest. (See Amazon's listings for rubber bands as an example.)
To describe an object that expands to a greater radius you would say it has more elasticity or better/larger expansion. This works best when comparing two objects with the same resting size:
Objects A and B are 2.5cm wide but B has more elasticity.
If you need to compare two objects with different resting sizes or need to explicitly note the maximum expansion size you would just use "expands to":
Object A is 2.5cm wide and expands to 4.5cm. Object B is 2cm wide and expands to 5cm.
If you only want to describe which has a larger size once expanded use "expansion", which keys on the definition of "the degree, extent, or amount by which something expands"
Object A has a larger expansion than Object B.
I don't know of any single word that serves the role you're asking (taking on -er); the closest is probably to say more [whatever], such as more elastic, stretchable, or expandable.
Edit, to take into account comment about actual state, rather than potential: Again, I can't think of an existing word that would take -er and mean what you are looking for. The closest phrasing I can think of with as few words as possible is "A is hyperexpanded relative to B".
I'm not just coining hyperexpanded myself; this word is commonly used to refer to lungs which have expanded beyond normal size. Some care might be needed when using this word with your circular bands, because there is the connotation of abnormality or damage. If I hear someone talking about a hyperexpanded rubber band, I'm going to think the band can't return to its original (undamaged) rest size on its own. That's why I included "relative to B". Still doesn't feel exactly right, but nothing better is coming to me.
It is difficult to get at what you are asking without more context, but for rubber bands, the two main properties people are interested in are elasticity and stretchability.
When stretched there is an "elastic limit" beyond which they will not return to their original shape, and further beyond that is a measurement of "tensile strength" where they break.
I would expect these limits to be described in terms of force rather than radius, but for a catalog etc. it may suffice to simply say "higher stretchability" or make up a term like "stretchability index of 4 inches" etc etc