That assessment isn't really accurate (well maybe for 3rd and 4th edition). Yes, AD&D characters get more HP's as they level (until level 10 where they start getting a flat 1-3 with no bonuses), but monster and spell damage for similar level creatures usually outpaced what PC's gained (meaning that, at best they cancelled out). More importantly, the assumptions of what HP's mean are different for each game. HP's in D&D have never been wound points. They represent the PC's ability to parry blows. In AD&D, if a hit doesn't drop you to negatives, it didn't really hit. It was parried, or failed to pierce your armor. Over time however, the onslaught would wear you down. You'd get tired (hence, lower hit points).
Now, in BRP this is handled by two systems, hit points (actual wounds) and defense rolls. So both of these systems have to be taken together to really analyse them against AD&D HP's. In my opinion, the ability to negate attacks (based on a skill that raises with time) completely makes up for the lower actual HP's. So BRP characters and AD&D characters are mostly equal in squishiness.