Anisotropic magnetoresistance (AMR) plays a critical role in spintronic memory and magnetic sensing devices. Although AMR is generally weak in bulk magnets, it is markedly enhanced in two-dimensional systems. However, its systematic evolution with dimensionality remains largely unexplored. Here we investigate the dimensional dependence of AMR in the van der Waals antiferromagnet ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ using density functional theory and Wannier-interpolated Boltzmann transport methods. Our results reveal a pronounced increase in the out-of-plane AMR as the sample thickness decreases, rising from 19.57% in the bulk to 42.93%, 52.17%, and 60.97% in the trilayer, bilayer, and monolayer, respectively. We uncover a mechanism governing this evolution: magnetization reorientation modulates the out-of-plane orbital hybridization between Bi and Te atoms, an effect that is strongly enhanced in thinner samples. This orbital hybridization variation directly tunes the spin-orbit coupling strength, leading to considerable band splitting that depends critically on the magnetization direction. Our work provides a unified microscopic understanding of dimension-dependent AMR and establishes ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ as a promising platform for designing tunable spintronic devices.

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• Anisotropic magnetoresistance
• Magnetotransport
• Spin-orbit coupling
• 2-dimensional systems
• van der Waals systems
• Boltzmann theory
• Density functional theory