Ingress Minimum Attack Efficiency
Even if what you're seeing with different bursters is the same 1% animation, you're not actually doing the same damage. If you noticed, you'll never see decimal numbers when firing an XMP, so what's actually happening is that you're doing a different amount of damage with the two different bursters, but those are both going to show as 1% because that's the nearest integer percentage that rounds both the amounts (you'll never see 0% when attacking). If you want to see the actual amount of damage, check the logs next time you do an attack.