The distribution of carrier density in an active layer of a vertical-cavity surface-emitting laser (VCSEL) produces a refractive index distribution, affecting the transverse-mode behavior of the VCSEL causing high-order-mode lasing. To achieve high-power fundamental-mode operation, we investigated the reduction in this effect by elongating the cavity. Experimental data show that the high-order mode started lasing immediately above a threshold in a lambda-cavity VCSEL with an active area large than 10 µm in diameter. By introducing a long monolithic cavity, high-order-mode lasing was suppressed. Theoretical results clearly show that such high-order-mode suppression is achieved because the effect of carrier-induced index change in the active layer is reduced by elongating the cavity.