Even for modestly-sized incandescent LV lighting installations, you have to worry about IR drop.
A 14AWG wire (fairly typical for these and recommended in the manual) is 8.3mOhm/m. For a 200W load, that's 1.4V of IR drop per 10m (small distance for a backyard), definitely noticeable
That's why:
* Most 12V lighting transformers, even professional ones have multiple 200W or 300W channels instead of fewer 600W or 1200W ones.
* Higher-end lighting transformers commonly have multiple taps for each branch (e.g. 14, 15, 16, 17, 18V) so that you can cancel out IR drop at the transformer.
* 12AWG is commonly sold for lighting and 10AWG often installed by professionals (that's a lot of copper to do a backyard - sometimes more expensive than the lighting appliances themselves).
* When building the topology, you try to group the lamps close to each other on each branch so that most of the IR drop is before the first bulb, you have even illumination of all the bulbs and you can adjust voltage at the first bulb by changing the tap at the transformer.
* LED lights are awesome...
So the Intermatic transformer from the OP is pretty standard for a lower end transformer. If it had just 12V, in most installations, it would be too dim. But then at 14V, especially if you have a light load or little wiring, bulbs are going to have a much shorter life. No easy way to deal with it.