When I select the current font size in the font size panel to increase or decrease it manually with the arrow keys of my keyboard, the reverse behavior happens from the expected behavior:

Pressing the UP key will decrease the font size;
Pressing the DOWN key will increase it.

Here's a screenshot...

Is there a way to make the program behave like other programs where UP means MORE and DOWN means LESS?

Describe your OS & version, the version of Inkscape and the installer you used, plus any special hardware like tablet/stylus or multiple monitors.

Sure thing,

macOS 10.13.16 High Sierra
Inkscape 1.0.1 (c497b03c, 2020-09-10)
Mac Mini and USB Apple keyboard, no tablet/stylus
Monitor Benq GL2460

I think that behavior is consistent with most dropdown menus. Compare with the line-spacing spinbox.

I don't know why text sizes have a menu, but it seems to be the norm across many programs.

You could request it be changed to a spinbox: https://inkscape.org/forums/beyond/how-to-report-bugs-or-request-new-features/

Can you give an example of a program you use where selecting text and pressing the UP key will make the selected number decrease?
I have never seen this behavior anywhere else but in inkscape.
Across every single Adobe app, UP means increase in selected number or percentage. Many people move from Adobe to Inkscape.

The logic is that we are navigating inside a suggested menu. Imagine a drop down menu with the labels:

Algeria
Belgium
Canada
Ethiopia
France

If you click on the menu (without expanding it) to navigate from canada to Algeria you would have to press "UP" twice... Imagine if you pressed UP and went from Canada to Ethiopia it would be counterintuitive and disruptive in a workflow.

Here the same thing happens with numbers. This is the Artboard size menu in InDesign (but the exact same behavior would be expected across all Adobe apps).
I am showing it in its expanded version, but if I simply clicked the value and used the up and down arrows to zoom in or out of my artboard, up would be Increase and DOWN would decrease.

It's a worldwide standard. Here is another example where up means an increase in value, Apple's Pages Text Editor. Unrelated to Adobe, different company - same behavior. I could find many others.
But here we have Inkscape really trying to stand out and confuse people with dozens of years of muscle memory.
Inkscape, by the way, is also inconsistent with itself, since changing coordinates of an object from the box will respect the "up means increase" standard, but not the increase of font size when changing text size.

Please clarify what you mean by "behavior seems normal on your end", giving precise examples. Thanks.

fabulousrice

But here we have Inkscape really trying to stand out and confuse people with dozens of years of muscle memory.

Nope. It's just a dropdown menu. Most dropdown menus navigate up/down with the arrow keys in relation to the screen. You can put in a feature request if you think it should be changed.

In your example you are pressing up to go Earlier in the list, therefore proving my point.
The orders are either smaller to higher or alphabetical.
The way this particular text box behaves in Inkscape is different from Inkscape's other boxes.

This is, in INKSCAPE, a behavior where the UP key increases the value selected in all the boxes.
This is, in INKSCAPE, a behavior where the UP key DECREASES the value selected in all the boxes.
Therefore inconsistent and a UX bug.
Apologies for how difficult the Keyboard Viewer is to interpret, now that Apple are making it darker. But hopefully you can tell which key is being pressed, up or down, in both screenshots.

Not a bug. It's a menu. If you prefer a spinbox, you can submit a request.