We present a new "pulsating box" setup to investigate particle acceleration in high-beta plasmas undergoing compression-expansion cycles. Our fully kinetic simulations show that particles are efficiently accelerated by magnetic pumping, producing nonthermal energy distributions with power-law tails. Numerical results are in excellent agreement with a generalized maximum entropy model that we derive, linking the power-law index of distributions to the injected energy. Our results could be relevant for understanding the origin of high-energy particles in certain high-beta space and astrophysical plasmas.