I was comparing it to HST WFC3 with a field of view of 160 x 160 arcsec^2 (https://www.spacetelescope.org/about/general/instruments/wfc...). Thats about 450 times larger than the MUSE narrow-field mode FOV.

I think you mean the high-resolution mode of the ACS instrument (https://www.spacetelescope.org/about/general/instruments/acs...) but that is broken and it was not repaired during the last HST service mission.

Actually what I was looking at was the field of view of an individual MAMA detector in the Space Telescope Imaging Spectrograph, that instrument has about ~ 100 x 100 arcsec2 of total field of view apparently.