Most drivers look like this:
But some have two layers, like:
Why the two-layer design?
Does it have any benefit over the one-layer one?
Different driver functionality and design requires different components and layout. For example most simple boost drivers (.8-1.5vdc) can be made relatively small. Then there's the plethora of 3.6-4.2vdc drivers which most seem to hover around the 17mm size. When you get into various buck drivers for higher voltage is when you see size increase, either the board diameter or, in the cases you posted above, by going double layer. The double layer is done to allow the diameter to stay at 17mm which is a very common pill size.
Due to higher voltage [ say 8.4v ] more components are needed , so say with P60 drop ins , as stated above , the driver dia is restricted to 17mm , most 8.4v drivers are 19mm or larger to accommodate the parts .. But with current technology they should be able to do it on a 17mm board , only problem might be cost ..