I think despite the Old World being drastically smaller in scale and limited in scope, it felt larger because a good deal of the fiction dealt with either very small scale characters (Brunner the Bounty Hunter, Mathias Thulman, Goetz of the Blazing Sun, etc) or one-offs who died by the end of the book or something. We got a much more street level view of things and it made the world feel more lived in and larger. We also saw a good number of stories that embraced the length of time evident in the background: series on Sigmar, the black plague, the war of the beard, the red duke, etc. The fiction was written to expand a setting, not necessarily advance a storyline.
AoS by contrast, and 40k now too, lean very hard into the larger storylines of the setting, which means that the same characters keep popping up. Makes everything feel smaller because, despite the infinite size of the realms, the same people show up everywhere.