Gallerists don't set the trends, they are set by the people that buy images. If people want big images to fill their walls then they buy big images. Granted gallerists prefer larger images because they can make more money off of them. Probably the biggest reason why this is prevalent in photography today is because of the ease of making large prints digitally. This was evident starting roughly ten years ago when the big printers were starting to get popular. I remember seeing this progression at PhotoLA every year. Since most photography is done digitally, printing the image takes only one click regardless of the size of it. Although photography may be a passionate hobby for most photographers, it is decoration for many of the buyers out there. They want something unique that goes with the sofa. It is also a business and people are in it to make money as hard as that is to believe to an amateur purist.
Collectors are less interested in size in general since much of what they own tends to sit in boxes not displayed on the wall. They are more interested in content. There are not a lot of serious collectors though.
If the trend of big prints continues it is going to create storage problems for museums in the future. I have read some curators lamentation over this. This limitation also means that museums will not be able to buy as many prints and shows will be more expensive due to the increased shipping costs. Also, many large prints are easily scratched because of the mounting methods.
Personally I like small prints. When I started photography I made everything 2x3 inches for the longest time. There is a beauty to holding a print in your hand and looking at it. I still don't like to print big. It is a pain to do in the darkroom unless you have a lot of space.
There are still people out there who generally print small. I don't think I have ever seen a large Kenna print for example. His work does predate digital though.