(Edit: I have changed the title. Originally, I only mentioned the weights of two weapons. In the comments, people are saying that there are more that seem wrong.)
According to user Drallo's weapon weights research here:
and the A3 kit dump here:
the TRG20 is too light, lighter even than the Sting or Vermin.
The MK18 may also be too heavy, being much heavier than the equivalent Rahim.
Edit - I should point out that I'm not saying that the TRG20 and the MK18 are too heavy/light compared to *each other*, but that they are compared to their *equivalents*.