Nicht nur Collins, auch das OED verzeichnet diesen Sinn und zitiert u.a. Shakespear:
minnow, n. and adj.
A person or thing of relatively small size, power, or significance.Sometimes opposed to Triton
(esp. after quot. a1616
) or Leviathan
1596 T. Nashe Haue with you to Saffron-Walden sig. N, Let him denie that there was another Shewe made of the little Minnow his [sc. Gabriel Harvey's] Brother.
1613 J. BoysExpos. Festivall Epist. & Gospel si. 22 Little children must bee caught and brought vnto Christ, and after these minumes are baptised in the sacred font, they must be catechised and further instructed in the principles of holy religion.
a1616 Shakespeare Coriolanus (1623)iii. i. 92 Heare you this Triton of the Minnoues ?
1796 C. Lamb Let. 27 May in Lett. C. & M. A. Lamb (1975) I. 5 My civic and poetic compt's to Southey if at Bristol—. Why, he is a very Leviathan of Bards—the small minnow I.
1847 Dickens Dombey & Son (1848) xlvi. 449 He..was considered, by the minnows among the tritons of the East, a rich man.
1906 A. Noyes Drakeii, in Blackwood's Mag. Apr. 70 Those five Small ships mere minnows clinging to the flanks Of that Leviathan.
1983 A. MasonIllusionist ii. 48 So they went fishing for minnows, and caught Leviathan.
1995 Daily Star 25 Apr. 35/1 The first round of the FA Cup when the mighty Northampton Town travelled..to play non-league minnows Littlehampton Town.