because, math. 
due to attrition each year (a certain amount of the fish die!), fewer and fewer fish from a given year class are around to keep growing. if the year class is big enough, more fish might reach 50″, and then it’s much more likely that some of those fish will survive to reach 54″ or 57″.
lots of variables contribute to fish survival of course, including water quality, forage, fishing mortality, latitude, etc., but don’t overlook the difference between stocking and natural reproduction when it comes to producing really big year classes. when the true giants are aged (or were marked to identify the year they were stocked) it’s generally true that they are from a particularly large year class.
Green Bay and Mille Lacs (and many of the other relatively new muskie waters in MN) experienced this kind of incredible success in their early stocking year classes, with extremely high numbers stocked plus high recruitment and growth rates due to rich forage and low competition. we’re probably starting to see a more sustainable balance in many of those lakes over the past 5 years.
a long-established, naturally reproducing body of water like Lake of the Woods is already in balance with predator/prey relationships and is dependent on a perfect storm of natural conditions to produce a large year class. that’s very different than what happened to Mille Lacs in the late 80s.
when you compare apples-to-apples (naturally reproducing lakes) it’s interesting how different they perform, and latitude (how far north/south) stands out as a key variable in growth. there’s definitely a “sweet spot” for ultimate size that involves steady growth over a long life – with the biggest component still being how long the fish lives. in general terms: as you go further south they grow faster, but die earlier. as you go further north they grow slower, but live much longer.
if you want the long version, look up this seminal research article:
“Growth and Ultimate Length of Muskellunge from Ontario Water Bodies”
John M. Casselman , Chris J. Robinson & E. J. Crossman
North American Journal of Fisheries Management
Volume 19, 1999, Pages 271-290
here’s some slides from that article illustrating the ultimate size potential on a range of Canadian waters, as measured in the mid/late 1990s.
– some of these lakes (including LotW) have since gone to a 54″ size limit. reducing angler harvest has increased the overall size structure, which would change the predicted ultimate size somewhat for the better.
– in particular, the Lake St. Clair data is pre-VHS. that disease clearly improved the size structure by reducing competition for food. as the lake has been repopulating since then, size is starting to slowly trend back down.
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