a) Temporal evolution of the X-ray loop-top source volume of the SOL2021-05-07T18:43 M3.9 flare. The 3D-derived volume (solid black curve) is compared to those derived from the traditional area-to-volume scaling law (Eq. (1)) based on the STIX 6–10 keV (blue curve) and XRT Be-thick (gold curve) images. The dotted black curves show the range within which the true source volume is twice as likely to lie based on the geometric uncertainties of the 3D reconstruction method. b) STIX- (blue curve) and XRT-area-scaled volumes (gold curve) normalised by the 3D-derived volume. The dotted lines show the area-scaled volumes normalised by the 3D volume uncertainty range.