She's turned from a timid archaeologist nerd type into info-trader / mind-flayer / super-model. Her breasts grow two sizes and her face changes shape, all within 2 years in-universe.
As such, she's the avatar of the change in direction between Mass Effect 1 and 2 / 3.
Pictorial evidence:

See, I have this little theory. Between the events of the first and second game, everything that's gone on made Liara change a great deal. The picture above shows that, as well as the increase of her... attributes.
I posit that Liara may well be the youngest matron in existence. And if not yet, very damn near. If she isn't one by the events of LotSB, then she is certainly in her matron stage by the end of the third game.
Aethyta mentions that asari grow bigger as they age. Benezia did, Liara will too(my Shepard is very happy about it
). You know, child of her mother. In fact... she already did.
So yeah, I pose that Liara is the youngest matron ever. Or will be very soon. Sure, this is firmly in the realms of headcanon, but somewhat supported by the evidence in game.