Older motorcycle engines have more inertial weight than most newer motorcycle engines. I will say most because there are incredibly old design methodologies that are still in practice by some manufactures. These engines retain all of the characteristics from engines that were being produced from the 1920's to the 1970's. Ural and Harley-Davidson come to mind as stalwarts in technology.
In general there are characteristics that define your experience.
1. Over-square Bore
Many newer MC ICE's have a piston diameter that is bigger than the stroke of the piston. GSXR, Rotax for Aprilia, Ducati, Yamaha R1, all of the performance 4-stroke single cylinder dirtbikes have this in common. A short stroke and very big 4 valve combustion chambers. The short stroke requires less inertia to get the crank to spin back around and complete the combustion process. The oversquare piston bore allows this.
2. Less Inertial Mass
This maps to above and is a key reason newer MC engines are nice and snappy on the throttle response and rev high quickly. You can also hear this type of revving with Formula 1 engines pre-2013. Here is a Renault engine in the Lotus playing Happy Birthday. Many of the same principles apply to modern F1 engines. SHORT STROKE, big piston, big valves, low inertial mass.
The XS that you mentioned has a much longer stroke than say a GSXR, CBR or Ducati of the last 20 years. It's a 3 cylinder with a big flywheel and a counterbalancer system to smooth out the engine vibration due to the long stroke and much bigger crank that is required to accommodate for the longer piston stroke. Take into account the larger offset journals on the crank, bigger connecting rod, larger crank counterbalancer, the counterbalance system and you have a lot of weight. Getting that weight to spin up quickly requires a lot of energy and the motor doesn't produce enough to make it nice and snappy in terms of throttle response. Getting that weight moving is a lot of work.
In a nutshell. Newer MC engines have less inertial mass. That's the simplest form of an answer I can come up with.