The girls looking, as you put it, as if they "could possibly exist in reality" may be for one of two reasons: One, there's a longstanding tradition of automobile and motorcycle manufacturers using women to sell their product. Two, the girls look like they're riding customized bar hoppers, etc. here, and there's the stereotype that women bikers (particularly Harley riders) are either thin and sunburned with bleached feathered hair or heavyset with huge busts. So Nguyen may be playing to that or poking fun at it.
Well, I wasn't referring to the bikes or their posing on them, but rather that, as greenmask says below, the women clearly have weight, solidity and muscles. They have waists in proportion to the rest of their bodies, and they are built like their backs could actually support their chests without being in pain on a regular basis. I don't know a thing about vehicles or ads for them, so you could be right about Nguyen playing on stereotypical biker models.
"Fairly cheesecakey"? More like VERY cheesecakey.
Well, considering the kind of cheesecake one is liable to see in comics, or how any number of other comic artists would probably do the "gotham girls pose on bikes" concept, this is really not all that cheesecakey. No one's cleavage or butts are hanging out, nobody's spine is unnaturally twisted or contorted, and they aren't ridiculously posed. Relatively low on the cheesecake factor, I'd say, despite it being an inherently cheesecakey concept.
And good GOD, what is wrong with Harley's legs? Anatomy fail.
I'm not seeing the fail here. Her right knee is bent toward us, and it looks like she's wearing big metallic kneepads.