Currently they are in bit awkward middle ground where they may or may not be able to lift a log (400lb) as many have carrying capacity in the 350-399lbs region. There isn't even a large difference in cost but there is a large difference in utility, especially now as the introduction of springs has made non-coastal settlement locations more practical.
Currently it's a roulette if the cow you are investing your ~80 squirrel hides in will lift a log, so practically you really should save scum to check it out. In addition cows aren't that common, especially in the eastern parts so poor rolls may result in none of the animals being suitable. The average size of cow (the regular, non small, non large one) should be increased a tad so that they will lift at minimum 400lbs.