You're right that the ideal is probably lighter than 180lbs. But you also have to consider that many things don't get smaller as the person gets smaller. Radio equipment, medical supplies, weapons, and ammunition don't get any smaller. Building fortifications, breaking down doors (as was brought up), and other such tasks don't get easier. Helmets and body armor get slightly lighter, but it's probably not proportionate. Also, food demand probably doesn't decrease proportionately with weight. So there starts to be a point of diminishing returns with smaller soldiers.
Just thinking about 130lb guys, I'd say most of them are either elite athletes or just skinny and weak. Now, that may just be a result of the American population, as you may be right that many gurkhas come in around that weight.
In the end, you're going to have to be more specific to get the right answer. What type of environment and terrain is the war being fought it and what type of campaign am I waging? What do my supply lines look like? Are my troops motorized, mechanized, or on foot? What kind of weapons do I have available? What amount of resources do I have to procure equipment, supplies, and weapons?