Ambient illumination changes cased by image render size - ?

[Sfumato]

Dear friends, I found myself in a strange situation not for the first time (and it seems to be only in 1.7.3, not in 1.6.9 or 1.7.2): the image was prepared for rendering on a notebook and had dimensions say 500x700. Then I usually take the parameters (m3i) to my more powerful graphic station, open, set dimensions, say 3500x4900, decrease raystep multiplier, stepwidth limiter, press 'Calculate 3D' and go to bed if Parameters OK in the below window and 'togo' isn't say 23hrs...
The image I saw in the morning lately is far from making my day: they are much lighter and I have to spend bunch of time adjusting light for > 17Mpx image once again. So I experimented a wee, rendered a 2770x3490 image and then reduced the dimensions to 693x872. As I had expected the image became darker (only global lights used, neither positional, nor lightmaps).
Please tell me what is the right way to change the light conditions (i. e. parameters) in case one changes dimensions of image rendered.
Thank You in advance,
Sfumato.

[Jesse]

Just changing the image size without changing the DEstop value (how near the raymarching stops infront of the object) changes the iterationcount of the objects surface, meaning there is more detail, other shapes, other colors.. and maybe more.

So there could be some effects that changes the light amount, like the color of the object getting darker or brighter, and because you changed the raystep multiplier and or the limiter, the dynamic fog is also changing (if enabled) what can lead to big light changes.

This is a general problem where no easy solution exists, you always have to adjust these parameters in the final rendering.

Or do the preview rendering with the same raystep multiplier etc and downscale the DEstop value like the imagesize, then you get a more similiar result (without guarantee).

[Sfumato]

Thank You very much, Jesse for substantial and prompt reply! Of course it's DE stop - now that You explained it it seems sobvious :-)
With kindest regards,
Have a lovely weekend,
Sfumato.