From equi- +‎ ponderous.

equiponderous (not comparable)

1. Having equal weight.
   • 1722, Edward Barlow, An Axact Survey of the Tide:

     A Method much different from our Progressive Conduct, whose Engine-Wheel is light, and equiponderous on all Sides, and every Ladle hath its Opposite for a Counterpoise

• “equiponderous”, in Webster’s Revised Unabridged Dictionary, Springfield, Mass.: G. & C. Merriam, 1913, →OCLC.