Hello i have a problem, whenever I make a screenshot it shows up in low res/quality I have all settings maxed 4k bike ect it looks great in game but still the screenshot looks like this

i know it doesn't look too bad but in game it looks way better the same happens in photoshop my pc is very capable so I don't know how to fix this please help
PC Specs:
CPU: Intel Core i7 9800X
GPU: 2X GTX 1080Ti in SLI (1 ACTIVE FOR SIM)
RAM: Corsair Vengeance RGB PRO 3200MHz DDR4 32GB
STORAGE: Samsung 860 EVO 2 TB
PSU: HX850 V2
OS: Windows 10 Pro
MONITOR: Acer Predator Z35P 4k
BTW i have a post-processing mod on