fullness, enough, pride, satisfy
盈 centers on a container filled to the brim: the state of being full, having enough, or being satisfied. The pride sense is a natural extension of feeling full of oneself.
盈 combines 皿 (dish, vessel) with a top component that likely represents overflowing or abundance, together suggesting a vessel filled to excess.
Picture a dish 皿 with something overflowing from the top 夃: the vessel is so full it can't hold any more, capturing fullness, enough, and satisfaction.
For エイ, imagine an 'A' grade dish filled to the brim: 'A' -> エイ, and the dish is perfectly full.