Note that ἴσα is neuter plural. Neuter because it is the adverbial use of an adjective. But, why plural?
The neuter plural form ἴσα appears two other places in the New Testament. In Revelation 21:16 in makes sense because the subject length and width is plural. The other verse is Luke 6:34. There it implies equal as getting back the full amount.
Note in the statement:
πατέρα ἴδιον ἔλεγεν τὸν θεὸν ἴσον ἑαυτὸν ποιῶν τῷ θεῷ.
(John 5:18, NA28)
ἴσον is singular because one characteristic of equality is referenced. Jesus spoke of God as if God were his intimate birth father, not his great...great grandfather or origin.
Phil. 2:6 has the plural ἴσα because Jesus is equal to God in multiple attributes, not just one. If you interpret ὃς ἐν μορφῇ θεοῦ ὑπάρχων
(Phil. 2:6, NA28) as referring to physical form, then you say God the Father has a physical body. But, God is spirit (John 4:24). Thus, in means Jesus has the nature of God.
“in the nature of God” This is the Greek word morphē which is used in several senses: (1) an Aristotlian sense of essence; (2) the sense of the nature of something, or unchanging essence of something (this is how the early Church fathers interpreted it); or (3) the outward form of something, as in the Septuagint (LXX). This does not mean that YHWH has a physical body, but that the attributes and characteristics—the very essence of God the Father—are evident in God the Son. It is another way of asserting the full deity of Christ.
Utley, R. J. (1997). Paul Bound, the Gospel Unbound: Letters from Prison (Colossians, Ephesians and Philemon, then. later, Philippians) (Vol. Volume 8, p. 180). Marshall, TX: Bible Lessons International.
Dr. Wallace gives the reason for no article with θεοῦ.
Phil 2:6 ... This is a debatable example. Wright argues that the article is anaphoric, referring back to μορφῇ θεοῦ. As attractive as this view may be theologically, it has a weak basis grammatically. The infinitive is the object and the anarthrous term, ἁρπαγμός, is the complement. The most natural reason for the article with the infinitive is simply to mark it out as the object (see “Article as Function Marker” for discussion of this usage). Further, there is the possibility that μορφῇ θεοῦ refers to essence (thus, Christ’s deity), while τὸ εἶναι ἴσα θεῷ refers to function. If this is the meaning of the text, then the two are not synonymous: although Christ was true deity, he did not usurp the role of the Father.
Wallace, D. B. (1996). Greek Grammar beyond the Basics: An Exegetical Syntax of the New Testament (p. 220). Grand Rapids, MI: Zondervan.