The two adjectives ploi+ko/s (here in the feminine nominative singular, evidently extracted from somewhere) and plo/i+mos (in the phrase quoted) come from the same root with different suffixes. See pi 1783 and pi 1805.
[1] The phrase plo/i+mos qa/lassa appears in the Ambrosian Lexicon (according to Adler) and ps.-Zonaras, but not elsewhere in this precise form. It might nevertheless have been generated by such passages as Theophrastus, Characters 3.3 (th\n qa/lattan e)k *dionusi/wn plo/i+mon ei)=nai) or Plutarch, Pompey 78.3 (ba/qos ou)k e)/xein plo/i+mon trih/rei th\n qa/lattan).