If you make a tiled walk in shower with the drain in the center, for water to drain properly the floor needs to be sloped towards the drain. So you need the floor to be an inverted pyramid. With large tiles this requires four extra cuts (pic source):
If you don't like this "X" look then a solution is to use a line drain, which avoids the inverted pyramid shape: the floor can have only one slope plane. Pic source. However now you have another problem, which is this huge expensive drain which accumulates a lot more crud than a small one in the center.
One advantage of small tiles is they can easily follow the shape of the floor so it is possible to build an inverted pyramid sloped tub, with the drain at the proper spot (ie, the bottom) and tile it (pic source):
That's the main technical (non-aesthetic) reason for using small tiles.
Grout lines are always slightly recessed. This has the drawback of accumulating water and growing mold, but the advantage is that it adds texture to the floor and it is much less slippery against wet feet. In a shower, it's definitely a good thing. If you want slip-proof large tiles, it's a good idea to look into the exterior tiles.
Now as DavidBooth says the main drawback of small tiles is there's so much grout. It takes longer to do it properly and it's more difficult to clean.
Therefore if you use small tiles for a bathroom floor, it's a good idea to use non-porous grout, either epoxy or acrylic, but not cementicious grout. These non-porous products are much easier to clean due to mold and algae not growing inside the grout, but only on the surface. Even if it's moldy, it's possible to clean it back to its brand new look without too much effort.
However non-porous grout is also harder to apply and more expensive, and with mosaic you need quite a lot of it. But it's definitely worth it, it saves so much more cleaning time in the long run.
You should make sure the installer does not push too hard on the grout, because that will make grout lines too "deep" and they will fill with water then get moldy. Perhaps it would be a good idea to visit a mosaic bathroom he did previously.