I bought a Ceradyne helmet in size L (similar to this one). However it feels very loose on I get neck pain if I wear it for a few minutes while moving around despite tightening the straps. Should I look into aftermarket pads / straps / suspension system ? Would they even help?
Is it an issue of a loose helmet or me not being used to the weight?

Thanks!

Your link's broken.  How thick are the pads in the Ceradyne helmet, and how soft are they?

Generally speaking, your helmet shouldn't feel loose, so something needs to change.  Might be thicker pads, might be that you need to go a size down.

For whatever it's worth, Ceradyne/3M's sizing and fit guide is here:
http://multimedia.3m.com/mws/media/1026563O/3mtm-ballistic-helmet-sizing-chart.pdf?fn=3M%20Ballistic%20Helmet%20Sizing%20Chart

Updated photo and ordered thicker pads. Fingers crossed.

Can also use the team Wendy Boa system

If you are having neck strain after a short period of wearing the helmet, that could be from a preexisting neck injury (diagnosed or undiagnosed) or a need for strengthening your neck and shoulder muscles. If you add NV to this set up this problem will get even worse.

As far as the slop in your fit, tightening the chin strap to solve this is not the best way to go about it. You may need to trade out some pads for thicker pads sub in either 3/4" or 1" pads as necessary. I even will add shims behind pads in my helmets to further reduce any play and improve comfort for long nights with a helmet on.
Ratcheting head band systems are awesome, but with out a proper fit of the helmet to start with it wont totally solve the slop problem. Additional tightening will help so much before it gives you a headache from the added pressure.

All of my helmets be it an HGU-56P, bump or ballistic have had padding moved, shims added for a uniform fit around my head, to be slightly snug. My head is slightly narrow at my temples and shimming here vastly improves comfort and greatly reduces slop when I'm using NVG's. Slightly snug is important as well because as the pads warm, they will become softer and any preexisting slop will get worse, very obvious when you put on a cold helmet. When I first put on any of my helmets, I am able to bend over and with out the head band or chin strap the helmet will not fall off of my head. The head band just augments the fit and further reduced motion of the helmet on my head, which is very important with NVG's. The chin strap keeps the helmet on my head during impact(s).

Personally I like more firm padding in my helmets, due to the warming issue above listed. Also when I add 1LB of NVG's and 7/8Lb of counterbalance and battery pack on the back, any slop is instantly magnified. Also with the added weight soft pads are crushed and your head is resting on the shell quickly, causing a headache. Also the padding is important for distributing the energy from an impact. Heck I replaced the stock comfort pads in my OpsCore with TW pads for the added firmness and just a better overall fit, the stock OC impact liner remains. Now the OC is a helmet that I can wear for days with minimal discomfort when this is the needed protection level.

Ballistic helmets suck to wear, they are my least favorite head gear, they are hot and heavy, but with proper fitting/shimming the discomfort can be reduced to where you will have minimal discomfort even after 10-12 hours with one on. At the same time neck and shoulder strength is important for injury prevention after you have ruled out a preexisting neck injury. But even with all of that in place a long night with a helmet and goggles on still is a pain in the neck! No pun intended.

Any suspension with a dial is going to help stability. Also wearing your hearing protection mounted to the rails with 3M adapters will help stability and comfort vs wearing under the helmet.