We present an optical-trapping scheme based on an on-resonance photonic nanojet (PNJ) excited using a plane wave. A two-dimensional numerical simulation demonstrates that a PNJ is enhanced through resonance with whispering gallery modes (WGMs) and achieves a larger spatial distribution, providing a stable trapping region for nanoparticles nearly four times larger than those of the WGM nodes without broadening by the PNJ. To further enlarge the trapping region, an asymmetric micro-resonator lengthens the mode field of the on-resonance PNJ. We also propose an effective method for addressing the nanoparticle-induced resonance detuning through exciting high-order WGMs of a larger-mode field volume.