As we all know, a great pump is never any indication of permanent muscle growth. A great pump can be a psychological boost, giving a false positive . But a pump, even in moderation rather than the skin bursting kind, may be all one needs for muscle growth. In any event, a pump will result with most any type weight training, from very slight to whatever limits. Old BB'ing saying. "More blood, more muscle", which is true to a certain degree.
As far as the pump goes, you can only get so much plasma/blood into a working muscle, in any workout. It's like having a glass being filled by a water faucet. Could run that water faucet for over an hour but only can get so much water into the glass, the rest will just run off. And the original amount of water in the glass stays the same.
Can be working on a great pump, things going really outstanding, but may fine the more sets being done, the muscles tend to begin to lose that pump. Any more sets are not encouraging, but may be reducing that original pump. Natures way of telling you that a muscle has had enough, no more is needed.
Personal observation only...sure many will disagree with me: Have noticed that gentlemen who train with
higher sets (15 to 20 plus) going for that extreme pump with lighter or moderate weights, tend to lose quite a bit of muscle size, given a flatter appearance, if forced to take as little as 3 to 4 weeks away from their almost daily split workouts. While others, who train with a more heavier approach to training, with fewer sets and training days, tend to hold onto their muscle gains. Of course diet, rest and life style will have a lot to do with any of this.
Good Luck.