Factors Which Lead To Rifle Weight: Some things work against others.
Number of Shots: Everyone wants as many shots as possible from a given fill of air. Just realize that the air for those shots must be contained inside the gun. The air reservoir is the single most heavy component of the gun. It's heavy wall extruded hydraulic steel tubing.
Power Level: The power for those shots is drawn from the air in the reservoir. The hotter you run, the more air you need. Power and a high number of shots per fill equals a large reservoir tube equals weight.
Note: Sometimes people fuss over the weight of pcp rifles. It's simply the expectations of high power and long duration of shots which catch up with you. Be satisfied with less power or less shots and you can make the rifle lighter and lighter.
Steel Vs. Aluminum: There are components which can be made of aircraft aluminum. Making the receiver of such material is one way to save weight. When I use aluminum, I anodize it. A wide degree of color, graphics, and texture is available. Going through the laborious and exacting procedure of Anodizing, and then coloring everything black seems alot like making a leaded glass window out of clear glass. Therefore; having put in the effort - why not print in color?
The Stock: The design of the stock greatly effects the weight of the gun. Big cheek pcs., palm rests, pistol grips, heavy square forearms, all add significant weight. This "look" is currently in fashion and dictates much of the weight. Figured woods are quite dense and are far more heavy than common grain woods.
The Scope: Big Optics add significant weight. Big optics require big mounts as well.
Style: Unless you are intrigued with "alternative" designs such as "bottle guns" (where the stock IS a bottle reservoir), then high power, high shot duration, traditional layout, and low weight is not a simple task. "Alternative" stock designs such as abbreviated stocks, skeletonized stocks, and self stocks (where the components of the gun are formed into shapes which pass for stock substitutes); are methods of reducing weight. The choices are always yours.
When all is said and done: Consider also, that weight adds stability to a rifle design and usually allows for more accurate shooting. When you attend a FT match, you'll see some of the darnest rigs. I can't tell you the times I've seen a really light weight rifle, with alloy parts all over it, AND an outrigger with archery weights hanging from the boom. Target rifles have always been heavy. The weight - especially muzzle weight - reduces the body's natural sway. With a heavy rifle, that little nerve "twitch" must move considerable weight. It won't move a heavy weight as much as a light weight. If you ever read about the off hand 200 yard rifle matches which were so popular at the end of the 19th century, you'll read accounts of people altering their guns in odd ways. Some hung a bar of steel under their rifle barrel. Some slipped a heavy pipe over their rifle barrel. I read of one who wrapped his barrel with cable the full length. These guys weren't nuts. They shot unbelievable groups, offhand, at 200 yards with these guns. They knew that weight could be a marksman's friend.
So, I've always been of the opinion that I will make the rifles safe and rugged. No aluminum pressure reservoirs here. I make what is required to do the job right, and the weight is usually just fine.