Electrical transport in non-centrosymmetric materials exhibits a nonlinear conductivity tensor component that generates second-harmonic voltages in response to applied electric fields. Harnessing this effect is essential for developing zero-threshold electromagnetic energy harvesters and broadband frequency upconverters, both of which require maximizing the nonlinear responsivity. Here, we demonstrate that inhomogeneously strained macroscopic slabs of the topological insulator Bi₂Se₃ achieve a record-high nonlinear responsivity of 10⁴ μm/V: a six orders of magnitude boost compared to its thin-film counterparts. We attribute this giant enhancement to a classical geometric nonlinear transport effect, which is driven by complex strain gradients within the bulk material. Notably, a similar geometric mechanism known as geometric nonlinear Hall was previously observed only in microscale-patterned structures. Our findings establish macroscopic strain engineering as a powerful route to enhance nonlinear transport, with immediate relevance to a broad class of layered systems, including transition metal dichalcogenides and moiré heterostructures.