The extreme magnetoresistance of compensated semimetals is governed by both the Fermi-surface geometry and carrier relaxation, but these contributions are difficult to disentangle in finite-size structures. Here, we combine longitudinal and Hall magnetotransport measurements with temperature- and angle-dependent Shubnikov–de Haas oscillations in single-crystalline Sb flakes grown by chemical vapor deposition (CVD), with thicknesses ranging from 110 to 783 nm. As thickness increases, the non-saturating MR at 2 K and 14 T rises nearly 30-fold, reaching 7.13×10⁵%, while the primary frequency F_α remains approximately 99 T without any systematic shift. A joint three-channel analysis of ρₓₓ(B) and ρ_xy(B) reveals that this evolution is driven by an increase in the mobility of a nearly compensated electron–hole pair, rather than by a reconstruction of the primary pockets. Angle-dependent measurements confirm the existence of a closed three-dimensional α pocket, and a reproducible high-frequency sector (335-377 T) is consistent with the electron β orbit of the L-point pockets in bulk Sb. Together, the transport and quantum-oscillation results show that thickness tunes extreme MR through dimension-dependent scattering while preserving bulk-like fermiology.