@sibsontheroad
I while ago I compared file sizes against quality levels to see where things "lined up". Inclusion of metadata adds to a small file size difference between Save As and Export As (or Export > Save for Web). Perhaps it will help:
Photoshop 2021
Save As
Save for Web (Legacy)
Export As
Save As JPEG, Level 12 Baseline Standard
3,382,351 bytes (3.4 MB on disk)
Save for Web (Legacy), Quality 100
3,035,739 bytes (3 MB on disk)
Export As, Great (7)
2,951,329 bytes (3 MB on disk)
Save As JPEG, Level 11 Baseline Standard
2,549,259 bytes (2.6 MB on disk)
Save for Web (Legacy), Quality 92
2,520,944 bytes (2.5 MB on disk)
Save As JPEG, Level 10 Baseline Standard
2,072,956 bytes (2.1 MB on disk)
Save for Web (Legacy), Quality 84
2,128,221 bytes (2.1 MB on disk)
Save As JPEG, Level 9 Baseline Standard
1,813,312 bytes (1.8 MB on disk)
Save for Web (Legacy), Quality 76
1,826,102 bytes (1.8 MB on disk)
Export As, Excellent (6)
1,835,030 bytes (1.8 MB on disk)
Save As JPEG, Level 8 Baseline Standard
1,597,234 bytes (1.6 MB on disk)
Save for Web (Legacy), Quality 68
1,579,870 bytes (1.6 MB on disk)
Save As JPEG, Level 7 Baseline Standard
1,245,822 bytes (1.2 MB on disk)
Save for Web (Legacy), Quality 60
1,301,819 bytes (1.3 MB on disk)
Export As, Very Good (5)
1,324,234 bytes (1.3 MB on disk)
Save As JPEG, Level 6 Baseline Standard
905,668 bytes (909 KB on disk)
Save for Web (Legacy), Quality 52
1,173,631 bytes (1.2 MB on disk)
Save As JPEG, Level 5 Baseline Standard
837,439 bytes (840 KB on disk)
Save for Web (Legacy), Quality 44
742,928 bytes (745 KB on disk)
Export As, Good (4)
695,320 bytes (696 KB on disk)
Save As JPEG, Level 4 Baseline Standard
786,204 bytes (786 KB on disk)
Save for Web (Legacy), Quality 36
658,904 bytes (659 KB on disk)
Save As JPEG, Level 3 Baseline Standard
740,953 bytes (741 KB on disk)
Save for Web (Legacy), Quality 28
596,724 bytes (598 KB on disk)
Export As, Fair (3)
590,963 bytes (594 KB on disk)
Save As JPEG, Level 2 Baseline Standard
753,850 bytes (758 KB on disk)
Save for Web (Legacy), Quality 20
546,622 bytes (549 KB on disk)
Export As, Poor (2)
563,817 bytes (565 KB on disk)
Save As JPEG, Level 1 Baseline Standard
709,518 bytes (713 KB on disk)
Save for Web (Legacy), Quality 12
437,863 bytes (438 KB on disk)
Export As, Very Poor (1)
498,396 bytes (500 KB on disk)
... View more