Before you read this, you should know that this started out as a topic in our Message Boards, in response to a prompt. This topic has been overdone, I know, but I liked my theory and Masamune told me to put it up anyway. This theory covers Baby Yoshis (Super Mario World's version VS other Yoshi games' version), Explosive Yoshi Eggs, and the Frog/Lizard controversy Ditto started in his theory.
On the baby yoshi thing-
As far as the young Yoshis in Super Mario World go, I would say that they definitely weren't "baby Yoshis" in the literal sense of the word... now, to get some facts to support it.
First of all, these younger Yoshis are referred to as "Yoshi's Friends." I'm not sure how they could be his friends if they were newborns, not even hatched yet (although conversely this could have just been a bad translation.) I remember being thoroughly surprised at the end of SMW when all of his "friends" hatched... as babies. The theory that they were magically shrunk to fit in works well, though it strikes me as a bit odd that eating things would counteract this magic. This can be explained however, if you attribute this effect to the same chemical properties the Koopa Shells have to allow Yoshis to breathe fire, grow wings, etc. This biological phenomena belongs to a whole different theory, so I'll not go into it here, but logically it can be used to explain the negation of the theoretical "shrinking spell" used to change the Yoshis into 'babies.' Of course, Starmen (and perhaps other power-ups? I never did check,) can mature the Yoshi instantly, which seems to imply that Starmen are even more potent wards of the spell, and therefore much more powerful than the counterbalancing nature of the Koopas (and other enemies.) This works nicely when you think about their effects on the Mario Bros.
The fact that these magically shrunken Yoshis look much different from "normal" baby Yoshis as seen in Paper Mario also support this theory... those Five had noticable arms and legs. It can, however, be argued that the five in Paper Mario were older than those in SMW, since those in PM talked, and showed these more developed limbs. (This does not explain why Yoshis grow so quickly in SMW, while those in PM seem to require parental guidance and nurturing just as any child would, though... and those in Yoshi's Story had just hatched and were formed just as those in Paper Mario were, so it is a flawed argument altogether, that )
Another piece of evidence to support that theory, Yoshi himself begins as a "shrunken" yoshi in SMW, when you first find his egg, then rapidly matures without needing to be fed. I assume this is due to his relatively short time spent under the spell compared to the amount of time the colored Yoshis (and the Yoshis you rescue from the Koopalings) spent inside their eggs.
And, of course, Yoshi himself does mention that Bowser himself trapped them in eggs, definitely implying some Koopa Sorcery is at work (though this was before Nintendo had really established the magical underpinnings of the Koopa Culture, with only this sparse hint and some Magikoopas inserted into a few areas.)
On the egg thing-
It is a well known fact that all Yoshis can lay the explosive types of eggs, whether adults (Yoshi's Island) or Babies (Yoshi's Story.) However, even if the eggs in Super Mario World are not actual Yoshi Eggs, but magical prisons created by Bowser, Yoshis still hatch from the same eggs that are used as weapons as evidenced by Yoshi's Story, and even the scene in Super Smash Bros' intro (which shows several Yoshis caring for a nest of Yoshi eggs.)
The obvious explanation for this is (and we're all mature in here, hm?) that unfertilized Yoshi eggs are volatile in nature, thus causing the explosive reaction, while fertilized eggs are cared for by a collective community of Yoshis. (The community spirit can be seen outside of the brief SSB clip by the apparent panic of even those Yoshis without obvious parental claims to the baby Yoshis in Paper Mario when they are lost. :P) This also, obviously, says many things about reproduction of the species; fertilization could not take place externally (as frogs, to get slightly back on topic, do) because the unfertilized eggs are so unstable when laid-- Thus, it must take place internally, and that is as far as I will take this path of inquiry. I believe that is more along the lines of a lizard than a frog (although there are lizards of both methods, I'm sure.)
About the Frog/Lizard thing-
Ditto raised some great arguments in his Mariology that argued for the cause of a dinosaur/frog hybrid species, and I can't entirely discredit it (nor do I want to try.) Suffice to say that Yoshi certainly is not a division of a frog, and if he is related to the species (shown by the bulbous nose/eyes, etc, as Ditto pointed out in his theory) it is a highly evolved form of the amphibian... (to the point where it is probably not an amphibian, but a lizard. ) Clearly, Yoshis do not show many qualities of your normal amphibian, since they do not have webbed appendages, and are in fact biologically far better suited for land travel. They do not have to remain near water to live, as amphibians would, and their offspring is in no way bound to the water for their first phases of life, with the eggs being hatched solely on land in nests (as dinosaurs did.) Although Yoshis are obviously strong swimmers, several real dinosaurs were, too... and as Masa points out, there are lizards that exhibit the qualities Ditto attributed to frog ancestry in his theory. It is easy to argue both sides effectively... patterns of evolution are hard to follow when you don't know much about the creatures there were to evolve from.