They do have big schlongs for their size.
I think there are mostly cock pics, vids are few.
In my humble opinion, missionary is not easy to achieve, even more so if u are a male.
Dildoggin in the this position is prolly easier with a dog the size of a basset.
I've seen chiuwawa vids and those small dogs the look like mops in mish position. I didn't see much humping going on.
Anyways, maybe someone else with actual experience in the matter and will chip in.