I think its some more sorts of headcrabs or rather more stages. One when the hosts body is falling apart. Becos they cant last forever. Then they leve the host and finds a bigger one(possible an other race) for exampel the original spieces behind the alien grunt. And that problably contiues until they host an gargantua. After that they becomes Big momma... But during HL1 its possible that many of the needed species are killed for making sure it dosent become more then needed big mommas. I also think that the big mommas now are made in a lab.
There are still some flaws in this theori, just so you know...