I counted the pixels and Siglent is only 10% more vertically.
In practical terms that's hardly any difference at all.
Indeed. The Rigol may look like it is pushing more information on screen but in reality the difference is minimal (and none if you take into account the Siglent screenshot doesn't show where you are in the acquisition data).
If you consider Rigol's "navigation" inset in the window frame to be part of the useful net information, it's 442 vs. 484 pixels, the 10% difference claimed by Fungus. If you just look at the main trace display, it's 414 vs. 484. That is quite noticeable in practice, at least for my taste.
And nevertheless Siglent fits more useful information into the bottom bar, e.g. the probe attenuation and detailed settings for each math function and zoom. And they fit in the top menu bar which gives you more direct access to many settings, where Rigol requires an extra tap on the "gearwheel" menu (plus some extra time to get your bearings once that menu opens).
I actually liked the Rigol UI on my DHO1074. The larger screen offers more room, so they do display math function details, and the "decorative" elements take a smaller percentage of the screen real estate. But for my taste, Rigol did not do a good job scaling the UI down to the smaller DHO800/900 screen. They should reduce the size of fonts and icons a bit, and remove or reduce the various trimmings, to leave more net area for traces and results.