The baby Yoshis in Super Mario World are not actually babiesSuddenly the egg hatched, and out popped a young dinosaur named YOSHI, who
proceeded to tell Mario and Luigi a sad tale of how his dinosaur pals who
were sealed in similar eggs by a group of monstrous turtles.
On the surface, this is referring to the gold Yoshi eggs that follow Mario after clearing a castle, but I theorize that it also applies to the red, yellow, and blue Yoshis in the Star World. Those Yoshis were adult Yoshis that were sealed into eggs by Koopalings. So it's not that they eat five things and grow up into big Yoshis, it's that eating five things somehow breaks the curse on them and returns them to their actual size.
Why do green Yoshis come out of the eggs big, then? Well, they actually don't. For a few frames when the egg first hatches, the green Yoshi is baby-sized, and then becomes big. Also note that this also holds true for all colors of Yoshis gotten outside of Star World in the GBA version. It seems there is something about Dinosaur World that breaks the curse right away once the egg is released from the block. The ones in Star World don't have whatever it is about the ambience of Dinosaur World, and so the curse still needs to be broken.
This is why the baby Yoshis in Yoshi's Story, Thousand-Year Door, and New Super Mario Bros U do not grow up no matter how many things they eat (sidenote: Does anyone know how many Yoshi Cookies you have to feed to the baby Yoshi on Yo'ster's Isle to make him fat?).
On a similar note:
Toads now manufacture blocksThe blocks in the original SMB were Toads that had been transformed by Koopa magic, as the manual states, and Peach turned them back into Toads after the game was over. But when Mario explained to the Toads how much the blocks had helped him in his journey, they set out to create a global network of blocks (this time not made from Toads (actually, though, maybe they are? Maybe when a Toad dies, they can have their body donated to the Block Ministry?)), so that whenever Mario needed to go on another adventure, he would have coins and powerups safely stored away for him (as it seems that no other species is able to open ?-Blocks). They also began researching new types of powerups. Some, like Super Mushrooms and Fire Flowers are naturally occurring (Super Leaves and Super Acorns may also be natural, or there might be some genetic engineering going on there (especially with the P-Acorn)), but many other powerups were created by Mushroom Kingdom researchers. The intro to New Super Mario Bros Wii gives us a glimpse at this, with the presentation of two recently completed powerups, and their initial distribution throughout the world.
Blue Toad and Yellow Toad were powerup testers for the Mushroom Kingdom research centerThey have gone through extensive training to match Mario and Luigi's physical skills, so that their testing of the powerups can be as accurate as possible.
Also, the block research lab in Woohoo Hooniversity shows us the Beanbean Kingdom's reverse-engineering of the Mushroom Kingdom's blocks, which they used to develop their own models. It's not clear whether they're doing this under the supervision of the Mushroom Kingdom, as a project for the Mushroom researchers, or if it's something they're doing on their own. The fact that the "Beanbean Ambassador" at the beginning of the game claimed to come with the intention of improving the relationship between the kingdoms suggests that their past may not be a particularly smooth one. The Beanbean block research may have been a defense operation.
Another thing I've been thinking about: How does the stork factor into Yoshi reproduction? Now, we've seen Yoshis hatching from eggs and we've seen Yoshis laying eggs, but we've never actually seen proof that those two are connected. It's possible that the eggs that Yoshis lay are actually a form of defecation, while the eggs that baby Yoshis come from are still delivered by the stork (I forget, was there any explanation of where the baby Yoshi egg at the end of Yoshi's Island DS came from?).