Here are the rules.
The Reapers are invading the Covenant before they first encounter humanity.
The Mass relays are in the Halo Galaxy.
Firepower and shields are equalized proportionally to their mass so two ships of equal mass will be equally powerful.
Please provide proof for your claims.
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Already had this discussion over on comicvine, this question has already been settled, Covenant godstomps. If we're talking about pre-Human-Covenant War Covenant, not long before they encounter the humans, then this is certainly not a discussion.