You want the maths?
pixel density is pixel per meter.
720 pixels wide doesn't mean that its covering the same number of meters as the 1080p.
( but 720 is height and so is 1080, not width).
http://www.rtings.com/images/resolutions-ultra-hd-4k-1080p-720p-dvd.png
1280 x 720 and 1920 x 1080
lets see if I can find a pre calculated example.
so say the camera is mounted two meters high
the field of view for 720p (12 camera is 80o, giving us a scene width of 3.36m
a 1080p has a wider field of view 118o, giving a scene width of 6.66m
so the pixel density on the 720 camera is = 1280 pixels / 3.36m = 381 pix/m
so the pixel density on the 1080 camera is = 1920 pixels / 6.66m = 288 pix/m
so the pixel density is higher on the 720p camera due its field of view compared to the 1080 cameras field of view.
the 1080 camera there is more data but its over a larger area lower pixel density, the 720 camera the data (pixel/density is higher) because the pixels cover a smaller area.
If you look at your garden and the camera can see your neighbours garden zoom in a little until your garden is at the edge of the camera (that's assuming the camera can be adjusted), you have increased the pixel density as the camera is covering a smaller distance from left to right but the number of pixels of the camera haven't changed.
There is lots more things to calculate but the field of view is down to the lens. if you can find some decent online calculators it may help.
(photography sites may help if you cant access those from CCTV sites).