Great question — answering as the UphillTech team:
I believe this is in reference to the clothing style changes of the time period. In the 1950s-60s the oversized Letterman sweaters were all the rage. As times have changed, the series has reflected that change. Outside of that, I think you may have a good point that it may be easier to animate shorter clothes, although not for the same reason. I think the more realistic and humanistic movements would be easier to portray this way. Outside of that, I don't have any references.
Full answer by Saxmeister: https://movies.stackexchange.com/a/72268 (CC BY-SA 3.0).
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