DSS blurring stars but Sequator is keeping them sharp?

[vpsj]

I'm trying to stack 240 light frames of Milky Way taken from quite a light polluted area.
I tried to stack them in DSS. The Milky Way seems to have been stacked okay, but the surrounding stars look like they've been deliberately dimmed or brushed over.
I thought there was something wrong with my frames but when I used the same frames in Sequator, it's working fine.

I've attached both the images below, and I have exaggerated them a lot on Lightroom just to see how much details I could pull out. Sequator one seems to have too many weird light bands(?) but I think I can fix them using an adjustment brush. But look at how many stars Sequator is able to show compared to DSS.

DSS image:  The settings I've tried on DSS and yet nothing changed:
1) Tried in Standard, Mosaic both

2) Tried both Sigma Clipping, and Auto Adaptive Average(These two were recommended by DSS)

3) Hot pixel detection and removal (tried it with enabled and disabled)

4) Nothing enabled in the cosmetics tab

5) Tried with and without Flat frames

6) Star Detection Threshold: Tried from a range of 50 stars to 300 stars. Even manually checked to see if DSS was picking any noise as star(it wasn't).

If I try with less than 30 light frames, DSS does an okayish job and the stars in the rest of the image still look like stars, but then the Milky Way has no details to pull out, unless I stack a lot more shots.

Can anyone please tell me what am I doing wrong in the case of DSS that it's doing such a poor job for the surrounding stars? If you guys need me to upload some Raw light frames I will do so as well. Any advice and suggestions are welcome. Thank you

EXIF info:
Camera: Nikon D3100, with 18-55mm kit lens

Exposure Settings: F/3.5, ISO 3200, 15s x 240 frames, 18mm focal length, no tracker

50 Darks, 50 Bias and 50 Flat frames.
DSS version: 4.2.3

[johneta]

Looks like field curvature to me. Others here could describe it technically better than me.

But I have had similar issues with wide shots of the milky way. I usually do deep sky, but the few wide field's I do are difficult.
If I use an 18mm lens (on APS-c sensor) the field curvature at the edges means that DSS cant stack the edges at all. It is actually not possible for it to do so, as far as I know, if you get too much movement between frames.  The outer stars are at physically different places on the frame if there is movement between images. 

I initially wanted to dither between images to reduce noise, but doing so moved the images between shots of course. -Then DSS couldnt stack it.
So I ended up not dithering and taking Darks (which I prefer not to do) to reduce noise. This meant I only took about 12 Light frames for the shot as my theory was, that I would get minimal movement over those 12 frames. It worked well for me!

Its not a problem over about 35mm in my experience, as longer focal length lens have less curvature.