I was about to write something like you said. I think that they are too heavy and hard. At the club where I play we use Xu Shao Fa ***
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1. One thing about balls is that the overall hardness can affect how heavy a ball feels. Either very soft or very hard can feel heavy. (Imagine kicking a partially deflated football, it often feels heavy even thought it is actually a bit lighter). Also of course diameter and smoothness affects the way the ball flies through the air and that can mean it feels heavy because you subconsciously figure out that you have to swing a bit harder.
2. A lot of people feel like Nittaku Japan 40+ is heavy but in fact it is by far the lightest plastic ball (I have weighed several types of plastic balls with a sensitive laboratory balance). In fact, unlike every other plastic ball, when the Nittaku Japan 40+ came out it was almost too light for ITTF 2016 standards (2.65-2.67 g)!!! Now they are right at where they should be, right about 2.70 grams. Remember that nearly all celluloid balls were around 2.65 grams, so that is part of why people say that Nittaku Premium plays most like celluloid, along with the bounce height).
3. The early Chinese seamed balls were all too heavy (2.80-2.85 g) but now they are within allowable standards (around 2.75 grams). The XSF balls are all at that weight too. The Chinese seamed balls (such as Double Fish and Butterfly 40+) have three main problems. They bounce low. They are not always very round. They still have durability problems, although this is improving somewhat.
4. By contrast, some people think that seamless balls like XSF have actually gotten worse. I see more complaints about egg-shaped balls and fracturing. In my view XSF is still durable but when it breaks it fractures catastrophically and shatters into pieces. Some people find it actually bounces too high (but that is far better than bouncing too low). It plays slower for sure. I still prefer them to Chinese seamed balls, but not as much as when they first came out.
5. The new Butterfly G40+ balls is an entirely different experience. Some people love it. I am not a big fan. Damn thing is completely smooth on the external surface and this affects the way it plays in a major way -- fast, no arc, and slides -- and this gets even worse with a little bit of wear. Same price as Nittaku Japan, so why settle? Also, of course, the bizarre sound, although I personally don't care so much about that.
6. DHS, currently makers of the world's worst plastic balls (sold under many trade names including Nittaku SHA), will soon be introducing something called the D40, which will be made of a different and better material, smaller seam (as with the good Nittakus) and better overall play. I will believe ity plays better when I see it. Someone reported that they had managed to obtain some from a DHS distributor and thought the ball still sucked, but who knows whether this is really true? I will buy some as soon as they are sold and report on what I find, and I certainly hope they are better. More good balls is good for the sport.