The first is just an ordinary eagle. Not much to say, other than that FC makes them a size smaller than the SRD, which seems logical to me otherwise you have them the same size as goblins and pech.
Eagles being Small actually always struck me as pretty appropriate. FC doesn't specify any exact measurement ranges for what fits in a given size category, but if we assume FC's sizes are basically the same as the SRD's (a reasonable assumption, I think) then eagles fit well as Small size. Full-grown eagles are pretty consistently 2-3 ft in body length, depending on species, with larger species getting over 3 ft. That puts larger varieties at roughly equal height for what I imagine a shorter goblin or pech would be, and comparable to items listed as Small size (eg, long swords are explicitly described as 3-4 ft in the Forge chapter, and rapiers and katanas are historically of comparable length as well). Plus, the rules for winged flight say that a creature is considered one size category larger when flying for purposes of Defense, and if we consider that eagles have a wingspan falling squarely in the Medium size range (eagle wingspans typically fall between 5-7 ft. depending on species) it makes sense to say that this should be their effective size for Defense when in flight, which would make their base size Small.
They'd be on the smaller end of the Small category, to be sure, and I could see an argument for their light weight pushing them down to Diminuitive when you consider both size and weight on the whole, but personally I've always felt Small was the best fit.