The Asus PA248CRV is a 24.1" monitor with a resolution of 1920x1200 pixels. It has about 94 PPI.
Your 27" 5K Retina iMac has a resolution of 5120x2880 pixels, and about 218 PPI. You are probably running it in Retina "looks like 2560x1440" mode. So in terms of physical text and object sizes, it would be "as if" you had an actual 27" 2560x1440 monitor with 109 PPI. But the Mac would draw the letter shapes with the greater precision that a 5120x2880 canvas affords.
Text is a little bit larger on the Asus monitor because 94 PPI < 108 PPI – and sizing is tied directly, or indirectly, to pixel counts.
Text is better-looking on the iMac for two reasons:
- The iMac has much higher PPI – and is using 4x as many pixels (2x as many horizontally, 2x as many vertically) when it draws each letter shape. This allows more accurate font rendering.
- Macs once used a special trick ("sub-pixel anti-aliasing") to make text look good on monitors that have regular PPIs (like your Asus monitor, or the old 27" 2.5K iMac monitors). My understanding is that this trick was rather messy and non-modular from a software maintenance perspective, and that Apple removed support for it from macOS several versions ago. So if your 2019 iMac is anywhere close to being up-to-date, it probably does not render text on a monitor like that Asus as well as an old Mac running a much older OS would have.
