For comparison, some details from me:
macOS 10.12.6 - Uptime: 2 days, 7:50
iconservicesagent 0.0 % cpu 2.26 cpu time 51.2 MB mem 0 bytes compr.mem 2 threads user
iconservicesagent 0.0 % cpu 0.10 cpu time 15.4 MB mem 1.3 MB compr.mem 2 threads root
kernel_task 0.9 % cpu 27:39.58 cpu time 1.24 GB mem 0 bytes compr.mem 140 threads root
My iconservicesagent is nowhere near your size. My Photoshop puts custom icons on most saved files, but other than that I don’t play as much with custom icons as I used to. But I remember the small 32x32 icons of Mac OS 7.x, and the huge icons sets of 512x512 that we have now. Caching loads of icons on the off-chance that they need to be shown again, will add up with time (if it works like: cached as needed from the first instance they were shown, i.e. Desktop and all folders that have been openend to reveal their content)
