The current size of Model Y (assuming the same overall length as Model 3 at 184.8 inch) is actually longer than any small gasoline SUV. Honda CR-V, Toyota Rav4 etc at 180.8 inch. So I think there are room for Tesla to go one class smaller than Model 3/Y.
We own a 2018 CRV. It has 10 more cubic feet of cargo space and I expect it to be more spacious than the Y. The Y is just a longer vehicle. The CRV has more of a square hatch/traditional SUV shape instead of a slope back/hatch like the Y. The extra length probably just allows the small third row option.
It's confusing and frankly I don't like the term "crossover". See the below (for example): Autotrader - page unavailable https://cars.usnews.com/cars-trucks/best-cars-blog/2016/09/suvs-vs-crossovers-whats-the-difference I've complained about the fuzzy definition and other issues I have w/how the media refers to crossovers at AutoNation: Gas Price `Freak-Out Number' Higher Than 2008 before. BTW, you can look at some older copies of that WSJ table via Wayback Machine. https://web.archive.org/web/20180119031548/http://www.wsj.com/mdc/public/page/2_3022-autosales.html is a table that covers all of 2017 in the US. You'll note that everything indented under "light-duty trucks" is considered to be that (e.g. pickups, crossovers, minivans, SUVs, vans, etc.) There are actually some vehicles that have too high a GVWR and are beyond "light trucks" and are considered medium duty trucks. See the table at https://www.fueleconomy.gov/feg/which_tested.shtm.
Definition: “A Crossover is based on a car's platform, (Model 3 in this case) while an true SUV uses the chassis of a truck. The result is that Crossovers use "unibody" architecture, meaning the body and frame are one piece, while SUVs use a "body on frame" design. (Like a Chevy Suburban) Although Crossover’s can have SUV type characteristics, it should not be considered one since it is primarily based on a passenger car platform. Early Crossovers did resemble small SUVs or large wagons. Crossovers today have ride, handling, performance and fuel / battery economy characteristics similar to the cars they’re based on and are only intended for very light off-road use, if ever.” Therefore the term Crossover fits the Model Y to a T !