As
@Stephen.R explained the same resolution in a smaller frame leads to higher pixel density, which makes the 24" version stand out. It's also one of the very few monitors with those 4k at 24" attributes. Meanwhile there's no shortage of 27" 4K displays, which makes the P2715Q less remarkable.
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Again, nothing wrong with that display. It's just that there is an endless supply of very similar displays available while the smaller P2415Q offers a somewhat unique size and therefor the same resolution at higher pixel density.