I find this interesting because my primary recently commented(and not in a negative way) that he could 'go deeper' with one of his secondaries...for me that simply means we have to be more creative with positions which is fun!
He is only average sized(as he calls it) but it's his ability to connect with me on the other levels of the relationship that give us our 'physical connection'. I know it's really hard not to compare yourself to him but if it makes you insecure on a psychological level then you need to communicate this to her and decide whether it's in both your interests that she is so open in her appreciation of him.
Its all a learning process and we all have different boundaries,so don't be afraid to express what is going on in your head. This is what I love about the poly relationship dynamic because it makes us into more loving individuals.