The carts are already plenty small. Yes, they could be smaller, but any smaller (without being downloads only) and they'd be difficult to handle.

For cost, they could likely reduce the pincount for new cartridges, by changing the number of data pins, but that doesn't preclude using the same slot. Reducing cost of cartridges is more effective than reducing the cost of the console. Reducing pin count would probably save more money than shrinking the small amount of plastic case.

For energy efficiency, maybe they can eliminate 3.3v and only keep 1.8v for new carts, maybe redesign the insertion detection pins to detect old and new.

The carts are shockingly big compared to Vita carts. The plastic housing with a PCB sliding around underneath always felt cheap to me compared to any cart from the late 80s to current.

they could have made them smaller the first time around, but I have to imagine they intentionally chose not to — we have to remember that they're also optimizing these things for children... so smaller isn't always better for things like swallowing (which is why they add a bitterant to the current cards)

Even at the size they are, I wish they were closer to GBA—cart sized.

You wouldn't vacuum a cartridge.