There are a number of things you need to consider.
Going back to the Great Crusade, it's implied that the Wolves were involved in the sanction of at least one other legion prior to the Heresy, which means that, even by then, they were probably a below average sized legion because of the likely casualties sustained in an astartes vs astartes war.
Then came the Siege of Prospero, where the Wolves took significant casualties fighting the Sons. Then Russ still dispatched an entire Great Company to finish the job. Following that, they were ambushed by a numerically superior Alpha Legion and then had to return to Terra to help fight off the retreating traitors.
Then came the Scouring, where the majority of the loyalist legions who weren't already pretty much wiped out got brutalised retaking what was lost during the Heresy. So by then, they would have been drastically reduced from their original numbers.
And they did have one successor chapter, the Wolf Brothers, which basically comprised of half the forces of the Wolves that were left (including gear, armour etc). Unfortunately for them, the Wolf Brothers fell to mutation and had to be disbanded.
And the Wolves are still considerably larger than other chapters. If I remember correctly, by the end of Battle of the Fang, despite nearly an entire Great Company getting destroyed, the Wolves still numbered around 2000 warriors, twice as large as a normal chapter.
Edit: Damnit, ninja'd multiple times. Also to add, the Wolves can't have successors because their geneseed goes ape shit with mutation if they're away from Fenris.
The human appendix. Proof of a higher power. A divine kill switch so to speak.
No one really likes a smartass, but people tend to like a dumbass even less.
Last edited by Chompy Bits; 08-28-11 at 06:30 AM.