I went from a 17" MBP to the 27" iMac. I seriously thought of getting the 21.5" iMac with the i7-8700 processor. However, Apple's outrageous RAM prices made me refrain from doing that. I also went to a local store to compare the sizes, and right next to each other, the 27" looked way more fun to use.
Yes, I sometimes have the feeling 21.5" would've been more reasonable - I used to work full-screen on most apps, on the 27", that almost feels like overkill at times (e.g. working in PowerPoint-editing a single slide in 27" is just crazy). So I have set most apps to only use e.g. 1/4 of the screen now, which of course makes the screen look more "cluttered" as when you see only a single file you're working on, and can be distracting.
However, in many cases, I'm glad I went big, e.g. when I'm working on a document and need to look at another for reference. No problem having 3 A4-pages open next to each other, that's really convenient. And editing photos on that huge, brilliant screen looks just amazing. The 17" MBP screen looks so small now in comparison.
I only wish developers would make better use of the format-screens are wider than they are long, yet you can have only one column of palettes/tools to each side of e.g. the photo you're editing (as it is you have a lot of "wasted space" on the screen, at least with the larger screens, but still have to scroll down e.g. in Lightroom to get to all the tools you need when working on one photo). Those stupid "ribbons" e.g. Office has make absolutely no sense to me, especially on a smaller screen (I also have a 13" MBP), where I'd like to see more of the page, but have a lot of empty space to the sides of my document, where they could put the palettes. Office 2004 had those floating palettes you could dock to each other to totally customize your workspace... where's that concept gone?