I may never understand Dreamland being a starter simply for how huge it is. Why not just make the five starters all mid-size stages? Green Hill Zone is smaller, but it's not Yoshi's Story or WarioWare-small.
Dreamland is...Dreamland-big.
If Delfino's Secret has smaller blastzones than Dreamland, then I'd approve of it becoming a starter instead of Dreamland. Plus, if the platform layout and pattern is based on Dracula's Castle, I don't see Delfino's Secret ever having the Norfair platform-camp problem.
tl;dr **** dreamland ****'s too big
Size is not the only thing that matters. Its how you use it. And thus, other factors such as platform layout, blastzone distance, and walled sides make a difference. Some characters with better recoveries like wider blastzones, but some of those characters also want a small stage for stronger zoning. Does the player want to ban for blastzones or for stage size? If their opponent wants a small stage, period, they will end up banning to either an average size stage (which is slightly beneficial for the character that likes either size) or to a small stage (slightly beneficial for the character that likes small for multiple reasons). If all stages were mid-sized, the small-preferring character would always be at a disadvantage unless against huge-preferring characters.
The large and small stages need to be available. If it at least forces a ban from the generalist character, the small/large-preferring character gets a chance to shift the bans toward a neutral stage (the smallest or largest of the middle-sized stages, for example). If all starter stages (or, in an extreme example, the only stage) were middle-of-the-road, some characters would never have a chance.
Furthermore, it is possible to ban for other traits while your opponent bans for your size preferrence. Lets say its, I dunno, Peach vs Falco. I could be totally wrong about this matchup but its just a theoretical example, so whatever. Falco needs to ban DL, but that lets Peach ban Battlefield and PS2, leaving Falco with his choice of flat stages, GHZ or SV. Assuming Peach does well on large but not small stages (which I know isn't the case, but its a theoretical example), Falco gets a small stage and Peach gets a flat stage in GHZ. Tada, neutral. And all because we have a tiny stage in GHZ. If that were FD, Peach could still ban platforms but Falco couldn't ban size.