The file size of Pokémon Sun & Moon were 3.7GB, Pokémon Ultra Sun/Ultra Moon were 3.6GB and had far less features and Pokémon. For context, here are the file sizes of Switch games:

Super Mario Odyssey is 5.7GB.
Legend of Zelda: Breath of the Wild is ~15.6GB (~50% of the Switch's default memory).
The Witcher 3 is 32GB (The Switch's entire default fucking memory, and the maximum size a current Switch cartridge comes in, larger ones are going to be made though).
Skyrim is 14.23GB.
DOOM is 13.4GB.
Smash Ultimate is 13.6GB.
Mario Kart 8: Deluxe is 6.7GB.
Splatoon 2 is 5.5GB.
Pokémon: Let's Go Pikachu/Eevee are 4.1GB each.
Super Mario Party is 2.7GB.
Pokkén Tournament DX is 3.2GB.

If you honestly think that Game Freak are removing features to try and save precious space while these games exist, you're fucking delusional. If you somehow think that Pokémon Sword/Shield are going to be bigger games than Skyrim, DOOM and The Witcher 3, then you have no sense of scale.If you think something has the space to cram DOOM, Skyrim and Super Mario Party into it and have ~2GB left can't fit every Pokémon, you are beyond hope. If you think that the biggest media franchise in the world is suffering from technical limitations, then you are out of your mind.

We are not in the NES era anymore, we do not have to squeeze every bit of space we can so a game can fit on a cartridge.

They should be honest man up, and said the real reason why there is National Dex on Sword and Shield:
They really want to hit the late 2019 release because this game will move consoles, and they cut the national dex to save dev time