You normally can’t use the Switch 2 cartridges on the Switch 1. You can use Switch 1 cartridges on the Switch 2 all the time, though. Apparently the Switch 2 games that work on the Switch 1 are those with upgrade packs available, and it’ll play the Switch 1 version, although I can’t confirm that. But for sure, Switch 2 exclusive titles don’t work on the Switch 1.
https://www.ign.com/articles/nintendo-fans-surprised-to-find-switch-2-edition-game-cartridges-also-work-on-switch-1
However, I wouldn’t plan around the chance the Switch 2 version of Z-A can play on the Switch 1 at this point. I did preorder the S2 version of ZA and I only have the S1, so I’ll be able to check on release.
Onto the main question, the games aren’t released yet, the 5gb and 10gb you’re seeing is an estimate at this point. I think the 1mb is because it’s in development, or it’s counting the access itself and not the content.
As reference, for BOTW, the upgrade pack is estimated at 9.6gb while the Switch 1 is at 14.4gb and Switch 2 ver is 14.5gb.
All that to say, it just isn’t accurate at this point. And honestly, in general, the estimates are never what they actually are.
