Adding mass to a tonearm


I would like someone to explain to me why adding headshell weights doesn't really alter the mass of the tonearm that much when figuring cart vs tonearm compliance. I have a Denon DL-103r and I keep reading that's it's for high mass tonearms. I also hear that adding headshell weights doesn't really alter the mass. What gives?

I want to try a DIY on my Pioneer PL-530 turntable tonearm where I mask off the arm such that only the chrome arm on the headshell side is visible and spray it with Plastidip. This would seem to add mass and resonance control. If it doesn't work the I can just peel it off.  
last_lemming
I bought various screws for my Reed 3p tonearm to add weight, because most vintage MM cartridges i have are too light, still usable, but the counterweight of the toneam is too close to the pivot, so those 12mm cartridge mounting thumb screws are 1.51g each (about 3g for pair) and made of the gold plated brass (non magnetic). Available here in various size and weight. Recommended!   
Soundsmith has  different headshell cartridge screws that changes mass.  Has anyone used these things?
New Pioneer PLX- 1000 comes with weights for both headshell  and rear of  arm .
Also smokes my 2K  TT.
SME III w/ 5 g wand comes with 2 weights for headshell each 2.2 g, so you can add extra mass of 4.4 g for less compliant cartridges. Convenient. 

Soundsmith has different headshell cartridge screws that changes mass. Has anyone used these things?


Yes, they use 3 types of metal and their weight is about 5g. The problem is, they are high and they raise tonearm’s centre of gravity.

I got better results with a heavier counterweight.