I have tried changing the GOP to 249, now the file is 5.87 GB (slighter lighter, but still very large for a 45 sec video). This is the export log:
plugin_mgr_get_object_file_plugins: error opening shared object file ‘C:\Program Files\Shotcut\lib\ladspa/AUTHORS’: “C:\Program Files\Shotcut\lib\ladspa\AUTHORS”: The specified module could not be found.
plugin_mgr_get_object_file_plugins: error opening shared object file ‘C:\Program Files\Shotcut\lib\ladspa/COPYING’: “C:\Program Files\Shotcut\lib\ladspa\COPYING”: The specified module could not be found.
plugin_mgr_get_object_file_plugins: error opening shared object file ‘C:\Program Files\Shotcut\lib\ladspa/readme.txt’: “C:\Program Files\Shotcut\lib\ladspa\readme.txt”:
[mov,mp4,m4a,3gp,3g2,mj2 @ 0000000005499e80] Using non-standard frame rate 59/1
[h264 @ 00000000055d6dc0] Reinit context to 2704x1520, pix_fmt: yuvj420p
[mov,mp4,m4a,3gp,3g2,mj2 @ 00000000055d8bc0] Using non-standard frame rate 59/1
[h264 @ 00000000099bb4c0] Reinit context to 2704x1520, pix_fmt: yuvj420p
[mov,mp4,m4a,3gp,3g2,mj2 @ 0000000009a707c0] Using non-standard frame rate 59/1
[mov,mp4,m4a,3gp,3g2,mj2 @ 0000000009a707c0] Value 1.000000 for parameter ‘seekable’ out of range [-1 - 0]
[h264 @ 00000000096f1c40] Reinit context to 2704x1520, pix_fmt: yuvj420p
[producer avformat] audio: total_streams 1 max_stream 1 total_channels 2 max_channels 2
[AVIOContext @ 0000000004a3eb40] Statistics: 1417969 bytes read, 4 seeks
[AVIOContext @ 00000000099bee80] Statistics: 1417969 bytes read, 4 seeks
[AVIOContext @ 0000000004a3ec80] Statistics: 1417969 bytes read, 4 seeks
[h264_qsv @ 00000000096f29c0] Initialized an internal MFX session using hardware accelerated implementation
[h264_qsv @ 00000000096f29c0] Using the constant quantization parameter (CQP) ratecontrol method
[h264_qsv @ 00000000096f29c0] profile: high; level: 52
[h264_qsv @ 00000000096f29c0] GopPicSize: 249; GopRefDist: 4; GopOptFlag: closed ; IdrInterval: 0
[h264_qsv @ 00000000096f29c0] TargetUsage: 5; RateControlMethod: CQP
[h264_qsv @ 00000000096f29c0] QPI: 4; QPP: 6; QPB: 8
[h264_qsv @ 00000000096f29c0] NumSlice: 1; NumRefFrame: 3
[h264_qsv @ 00000000096f29c0] RateDistortionOpt: OFF
[h264_qsv @ 00000000096f29c0] RecoveryPointSEI: OFF IntRefType: 0; IntRefCycleSize: 0; IntRefQPDelta: 0
[h264_qsv @ 00000000096f29c0] MaxFrameSize: 0; MaxSliceSize: 0;
[h264_qsv @ 00000000096f29c0] BitrateLimit: ON; MBBRC: OFF; ExtBRC: OFF
[h264_qsv @ 00000000096f29c0] Trellis: auto
[h264_qsv @ 00000000096f29c0] VDENC: OFF
[h264_qsv @ 00000000096f29c0] RepeatPPS: OFF; NumMbPerSlice: 0; LookAheadDS: 2x
[h264_qsv @ 00000000096f29c0] AdaptiveI: OFF; AdaptiveB: OFF; BRefType: pyramid
[h264_qsv @ 00000000096f29c0] MinQPI: 0; MaxQPI: 0; MinQPP: 0; MaxQPP: 0; MinQPB: 0; MaxQPB: 0
[h264_qsv @ 00000000096f29c0] Entropy coding: CABAC; MaxDecFrameBuffering: 3
[h264_qsv @ 00000000096f29c0] NalHrdConformance: OFF; SingleSeiNalUnit: ON; VuiVclHrdParameters: OFF VuiNalHrdParameters: OFF
[h264_qsv @ 00000000096f29c0] FrameRateExtD: 1; FrameRateExtN: 60
[mov,mp4,m4a,3gp,3g2,mj2 @ 000000000a05f880] Using non-standard frame rate 59/1
[mov,mp4,m4a,3gp,3g2,mj2 @ 000000000a05f880] Value 1.000000 for parameter ‘seekable’ out of range [-1 - 0]
[h264 @ 00000000096f1780] Reinit context to 2704x1520, pix_fmt: yuvj420p
[mov,mp4,m4a,3gp,3g2,mj2 @ 000000000b065040] Using non-standard frame rate 59/1
[h264 @ 00000000055cfe00] Reinit context to 2704x1520, pix_fmt: yuvj420p
[mov,mp4,m4a,3gp,3g2,mj2 @ 00000000055d6bc0] Using non-standard frame rate 59/1
[mov,mp4,m4a,3gp,3g2,mj2 @ 00000000055d6bc0] Value 1.000000 for parameter ‘seekable’ out of range [-1 - 0]
[h264 @ 00000000055cf940] Reinit context to 2704x1520, pix_fmt: yuvj420p
[AVIOContext @ 00000000099bbb40] Statistics: 1417969 bytes read, 4 seeks
[producer avformat] D:/OneDrive - University of Glasgow/videos/Motorbike 31.05.2020/GH030113.MP4
checking VFR: pkt.duration 1001
[h264 @ 000000000a117940] Reinit context to 2704x1520, pix_fmt: yuvj420p
[mp4 @ 0000000004a3e740] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead.
[mp4 @ 0000000004a3e740] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead.
[h264 @ 000000000a117940] Reinit context to 2704x1520, pix_fmt: yuvj420p
[mp4 @ 0000000004a3e740] Starting second pass: moving the moov atom to the beginning of the file
[AVIOContext @ 000000003bec0c40] Statistics: 6309154228 bytes read, 0 seeks
[aac @ 00000000096f1300] Qavg: 63219.254
[AVIOContext @ 00000000096f1140] Statistics: 4 seeks, 48139 writeouts
Completed successfully in 00:07:06