The two Main Street USA streets are exactly identical in width and length, from curb to curb and from Train Station to Castle. The Hub streets are also identical, with the same distance from Casey's/Coke Corner at the end of the street to the Castle wall across the Hub. Although WDW obviously now has additional detour side-paths to bypass the Hub after its recent reconfiguration, and once you get beyond the Hub the buffer areas between the lands are larger at WDW, but that doesn't impact the parade route and viewing area.
The Disneyland parade route's narrowest spot is Matterhorn Way, which is slightly wider than the Liberty Square bridge. The rest of the route through Disneyland's Fantasyland is very wide, with tiered viewing terraces and bypass walkways around the parade viewing area. It's wider and has more viewing capacity than the WDW section of the route through MK's Frontierland, with gentle curves throughout the route instead of the 90 degree turn from Liberty Square into Frontierland.
Most of those widened streets and custom built parade viewing spaces at Disneyland were built in 1997 to get ready for the truly massive Light Magic floats. Light Magic was a giant flop that only lasted one summer, but the parade infrastructure changes to Disneyland continue to pay off two decades later. Thus, Disneyland's floats can be larger.