In “low/medium data usage” network, the RA is equal to the LA.
But this doesnt mean the GSM cell is bigger than the GPRS cell. It just mean that n cells which are located in one LA are sharing the same RA.
As a consequence, PS Paging are “broadcasted” within the whole LA.
By using 2 RA per LA, the PS Pagings for one MS are broadcasted only within “half” the LA. But as a drawback, the MS are going to perform Routing Area Update when they are changing RA. In other words there is additional RA border which is running through your LAC.
In the previous configuration (1 RA = 1 LA), the RA borders run along the LA borders.
Due to the low amount of PS Pagings, it is more convenient to keep the 1 RA per LA approach.