From memory i recall that 765 is offered the large file, presently that's 'JCV_LaneAssistFull', D3060200A.jcv, which indeed has 866,660,352 byte size or ~866MB as you said. If you compare that to the 'JCV_Full-Screen' at only 26,384,935 bytes [D3153200A.jcv meant for small flash size 1xxx devices] and 'JCV_LaneAssistMedium' at 274,515,696 bytes [D3080200A.jcv] you can get a relative estimate of the amount of images available proportionally.

Have a look what's presently onboard and what 'name type' is listed in the GarminDevice.xml file, it'll probably be 'PreProgrammedLaneAssistFull' with a file name of D3060xxx0. But as i said even if it's got a smaller file loaded presently it can still use the large one anyway provided the map version and jcv file match. The *.JCV file names do tend to advance numerically with successive map versions by '10'. Read this: [Only registered and activated users can see links. ].