I'm just playing with the Streaming in the Capture tab. I run my ASI1600 at ~10fps, and have selected a 184x188 area aroung Vega (at daytime ), and recorded a 30s 16bit ser movie.
However. looking at the file, it has a size of 4 Gigabtye. Playing it with ser-player does show the size as 184x188, 282 frames, but only shows noise.

It looks like the recorder is ignoring the ROI and saving the full frame, and from the size of the file (4621734274 ~4656*3520*282) that seems to be 8bit data

The same seems to happen when in 8bit mode. The framerate is higher, it now records 362 frames in 30s, and the generated file size is 5.6Gb.

I don't think ROI is implemted for SER files, need to look into this unless someone cares to look at it first.

Hmm, I think that doesn't depend on the format itself. IIRC it is just a header and then a binary dump of the data stream. And the created header does match the selected region. For displaying in EKOS it does the right thing, sending the selected area data to the viewer. It's just that the data saving seems to ignore those settings and send all data to the writing unit.

Maybe I should mention, I run INDI on a separate computer at the telescope. EKOS is on my laptop. The streaming write is stored on the INDI computer directly, not sent to the laptop and stored there. So I'm not even sure if that is an indi or an EKOS issue
Edit: And maybe also relevant, this is with an ASI1600 camera.

Just dumping some info here that might help people looking into the issue (I'm afraid I'm not ablte to do so )
I opened the .ser file in IDL (data reduction software). I tried to read the data as 8bit, but this is just noise. So I read it as 16bit unsigned with a 4656x3520 size, and got the attached view:

So it is actually 16bit data that gets stored, but only the first half of the frame. Probably the copy buffer still counts in bytes for 16bit data? Indeed, the 8bit SER file I recorded does contain the full frames (4656x3520@8bit).

So to conclude, saving streaming data (from ASI cameras) does always save the full frames, even if only a subarea is selected, although the header says the size is that of the sub-area. And for 16bit data only 4656x3520x1 bytes (=half a frame) actually gets stored.