I think she was just comparing them purely for large cocks because while a vagina is initially easier to penetrate, to get full penetration from a big cock, it's easier to penetrate a girl's arse fully and without pain for her compared to a vagina where the guy will likely bump into her cervix which is painful for her and means he can't be balls deep.
Nonetheless, I'd to be scared to see the size of her shits 
In part that, but in part knowing about the muscular anatomy, the anus is easier to dilate than the vagina and is limited by the pelvic bones, while the vagina is limited by other factors. Depth is limited by the cervix in the vagina and the second sphincter in the rectum that leads into the colon, that just tends to be more.
But like you say a big limitation is the initial entrance, however if a vagina isn't warmed up and happy, that'll be tough to get into as well. The vagina can expand a lot with arousal.
Also, the size of my shits are not even remotely impressive. I'd wager your thumb is thicker than any shit I've had in the last... forever. Born without a functioning digestive system, although it has it's upsides, I'll never be obese.
You probably right.
TS is missing a crucial fact.
Anal sex is good for a man because the ass is tight.
If her ass is loose and can take up almost a can of beer..what's the point then? 
She shouldn't stretch her asshole even thought she likes it 
You do realise the arsehole is a muscle? Like the vagina. Sticking something large in it once in a while with proper warm up won't make it scarily loose. Can actually in fact tighten it up once you let it relax back. Plus kegals! Makes everything tight. Slow steady stretching can actually tighten muscles.
The whole old gay men stick tampons up their butt due to anal fucking is a myth. The sphincter is only damaged if it's stretched too far too fast, or just too far. I know my limits, I know my warm up time, so I stick to things much smaller generally. And those that do deal with incontinence - same rates among straights, gays, lesbians, comparatively male and female, often shit just slips out when you're old. Nothing to do with buttfucking.
As for why I enjoy it, well sure, I might not have a prostate, but the nerve ends send fabulous signals when a cock is in. Something insanely large doesn't feel good, but fetishists pay handsomely to just watch a big toy go in.