ffmpeg QP: AMD Radeon (Vega7) hevc_vaapi results in lager files than hevc_nvenc - why?

Question

I've switched from an RTX 3070 to an AMD Radeon Vega7 APU (4600G/5600G). I'm trying to tweak the ffmpeg parameters, so that the transcode output results in a similar quality AND file size. hevc_vaapi with -qp 25 or greater results in worse quality and / or larger files.

Why am i failing - and is it actually me who's failing? ;)

Here are my command lines

NVENC (edit: i've got these parameters from EposVox's GitHub scripts, look him up on YouTube, he knows what he's doing - i've altered them to support subtitles)

ffmpeg -hwaccel auto -i "$1" -map 0:v -map 0:a -c:v hevc_nvenc -rc constqp -qp 24 -b:v 0K -c:a aac -b:a 256k -map 0:s -c:s copy "ff_out/$1.hevc_nvenc.qp24.mkv"

VAAPI

ffmpeg -vaapi_device /dev/dri/renderD128 -hwaccel vaapi -i "$1" -map 0:v -map 0:a -vf 'format=nv12,hwupload' -c:v hevc_vaapi -qp 24 -c:a aac -b:a 256k -map 0:s -c:s copy "ff_out/$1.hevc_vaapi.qp24.mkv"

and a directory listing for file size comparison

-rw-rw-r-- 1 user user 1,8G Apr  2 04:01 eval2.mkv
-rw-rw-r-- 1 user user 374M Feb 16 18:17 eval2.mkv.hevc_nvenc.qp24.mkv
-rw-rw-r-- 1 user user 787M Apr  2 04:19 eval2.mkv.hevc_vaapi.qp20.mkv
-rw-rw-r-- 1 user user 709M Apr  2 04:33 eval2.mkv.hevc_vaapi.qp21.mkv
-rw-rw-r-- 1 user user 641M Apr  2 04:47 eval2.mkv.hevc_vaapi.qp22.mkv
-rw-rw-r-- 1 user user 522M Apr  2 05:01 eval2.mkv.hevc_vaapi.qp23.mkv
-rw-rw-r-- 1 user user 483M Apr  2 05:15 eval2.mkv.hevc_vaapi.qp24.mkv
-rw-rw-r-- 1 user user 443M Apr  2 05:48 eval2.mkv.hevc_vaapi.qp25.mkv
-rw-rw-r-- 1 user user 2,7G Apr  2 04:07 eval3.mkv
-rw-rw-r-- 1 user user 532M Feb 23 19:46 eval3.mkv.hevc_nvenc.qp24.mkv
-rw-rw-r-- 1 user user 1,5G Apr  2 04:23 eval3.mkv.hevc_vaapi.qp20.mkv
-rw-rw-r-- 1 user user 1,3G Apr  2 04:36 eval3.mkv.hevc_vaapi.qp21.mkv
-rw-rw-r-- 1 user user 1,2G Apr  2 04:49 eval3.mkv.hevc_vaapi.qp22.mkv
-rw-rw-r-- 1 user user 936M Apr  2 05:02 eval3.mkv.hevc_vaapi.qp23.mkv
-rw-rw-r-- 1 user user 851M Apr  2 05:15 eval3.mkv.hevc_vaapi.qp24.mkv
-rw-rw-r-- 1 user user 762M Apr  2 05:48 eval3.mkv.hevc_vaapi.qp25.mkv
-rw-rw-r-- 1 user user  15G Mär 28 06:49 eval4.mkv
-rw-rw-r-- 1 user user 5,7G Mär 28 07:05 eval4.mkv.hevc_nvenc.qp24.mkv
-rw-rw-r-- 1 user user 9,7G Apr  2 07:23 eval4.mkv.hevc_vaapi.qp24.mkv
-rw-rw-r-- 1 user user 8,7G Apr  2 08:15 eval4.mkv.hevc_vaapi.qp25.mkv

I've tried varying the -qp parameter and expected a similar file size.