My default mouse pointer size was too small, and I increased it one notch. Fine. But now the text cursor (shaped like an "I") is huge. Is it possible to increase the pointer size without increasing the cursor size?
It is possible. Cursors are square bitmaps with a part that is visible, such as the arrow, and the rest which is competely transparent. Cursor sizes depend on two things: the size of the bitmap for the cursors, which is always the same for every cursor in current use, and how large the visble part is within the bitmap.
You can see the differences in visible size whilst the underlying bitmap doesn't change by looking at the regular, large and extra large versions of the default cursor scheme using the mouse control panel. I covered cursor sizing in an earlier post
reloading mouse cursors and changing their size
If you're happy with the size of your pointer (and hand) cursors, then the obvious thing to do is to change the I-beam (Text cursor in control panel) and probably 'precision select' to [Use default] . That means you'd be using the same cursors for I-beam and 'Precision Select' as the default settings. These are inverting monochrome cursors. However, they would be 50% larger, as you will have changed the underlying base cursor size from 32 to 48. If they are still too large, then you need specific cursor files for those two. i.e. the non-transparent part of the image needs to be smaller. I did once try hacking one up, but I'd have to search for it.
Note that the "one notch" larger cursor isn't really much larger than the default extra large cursor. It does have thicker lines which makes it easier to see on high dpi monitors. Which was why I started using them. A downside is that if you make any change to cursors using the settings app, it rewrites all the cursors and resets to point to them. It's also difficult to back them up. A simple thing you can do is to save the scheme, and then save your current theme including cursors. If you import the theme it will use cursors stored with the theme which won't be overwritten.