It CAN happen, personally I wouldn't let it. Huge rams will mate with tiny ewes, and tiny rams will breed huge ewes if they are left to their own devices.
A large ram with a small ewe is almost certainly going to result in lambs which are really too large for her to carry and can cause severe dystocia during birth. Worst case scenario is that the lamb (and it usually a huge singleton which causes the trouble) is too big for her to delivery naturally and she will have to have a Caesarian. A small ewe who does give birth to her lambs naturally will have the huge burden of trying to supply them with enough milk.
I think the lack of detail in the literature is down to the fact that folk don't allow these matings.
It's not really recommended to put a larger male over a female for breeding any animal. However in mule breeding programs in the UK it is usual to breed shetland ewes to a blueface Leicester ram. The biggest thing in my mind is the size difference of lambs at birth. A lot of the bigger breeds are bred for larger birth weights. If my ewes average 6-8.5# lambs birth weight I dont want to breed them to a ram from a flock that averages 9-13# lambs birth weight.
One thought is to bring in a already crossed ram (half icelandic, half suffolk or hamp etc) or bring in a ewe of that breed.
Another idea is to get a small ram. I know someone with shetlands and she got a border cheviot ram that was well built but among the smaller of the group of ram lambs. She uses him for meat lambs crossed on the shetland ewes she hasnt chosen for her best shetland rams for the registered stock.