Mass lock is entirely based on the mass of the ship, and not biased to one side or the other. Simple numerical comparison. An adder cannot ever mass lock an asp.
I recently did some bounty hunting in my python, and I didn't have too much of a problem. I have a 6A or 6C shield generator (can't recall which off hand) with two shield boosters and two point defense. I went one on one with a Dangerous ranked FDL and won. Albeit, I did have to head back for repairs (hull at 43% by the end), but I won. Had a tangle with an Elite ranked fed gunship a little later, he had at least one missile rack and at least one thermal shock weapon (I got the notification from my ship). He didn't quite crack my shields even though I didn't have any pips to sys for most of that fight (needed the engines and the firepower more). I don't think any of his missiles even hit my shields anyways, the PD took care of that. I have yet to see an adder that required me to even spool up my MCs, as my pulse lasers are almost always enough to deal with the smaller ships.
