Well, one of the main advantages of going halfling at those classes would be the fact that you can ride a mount anywhere. In doors, in caves, etc.
So staying medium is in your interest.
As for flying...a bit more argument, but you need to take specialized ranger archetypes for that...but again, there aren't any with fly locked behind a wall locked in with their size (except hippogriff from that one archetype, I suppose- that would be large by default anyway those; also a couple are small creatures, so that size size dependant I guess, but not relevant for the conversation).
There is little reason to grab large, since again- medium lets you fit into places. Sure, extra str for carrying, but not that is it (and most have at least enough to carry a halfling anyway, and they get scaling str buffs as ACs)
Sidenote- Am I misunderstanding something about the math of mounted builds? Where does this +24 damage come from?
While I'll readily admit to the bit about the sword and monstrous mount, I am not sure what you are talking about with the roc.
Are you referring to the 'bird' option on the standard ranger list?
Cause that is an extremely hard pill to swallow when the animal companion list has a literal 'bird' entry, separate from the roc.
This has of course prompted me to look at some threads about the subjects and...honestly, I couldn't find reference to anything inclusive enough to agree to the idea that rocs are an option for rangers. But I am, again, honestly curious- where are you reading it as such? Is this from some publication I missed?
Anyway, going back to confirmed things- it takes two feats in order to get a flying monstrous mount (since both list that as an option under the 'mastery' section, meaning you need Monstrous Mount Mastery as well). While flying is fantastic, you need to spend so many feats (when you already ahve to focus on mounted feats and combat feats) and you are also trading out the previously mentioned advantage of medium mounts. So it should at least take serious consideration (there is narrow frame...but that just gets you into a room- it still makes movement hard, which hampers the all important charges)
SRD....
http://www.d20pfsrd.com/classes/core-classes/druid/animal-companions#TOC-Ro c
Roc
Rocs taken as animal companions by druids or rangers are typically newly hatched birds—a baby roc is the size of a person and ready for flight and hunting within minutes of hatching. Unfortunately for druids seeking animal companions of legendary size, an animal companion roc is limited to Large size—still large enough for a Medium druid or ranger to use the flying beast as a mount.
And as I've said, the discussion over the rules ahve not been conclusive on whether that expands the ranger list, or simply acts as fluff or forward thinking (such as predicting their future releases of things like the beastmaster archetype)
I know that might seem shallow to you (and I have little belief people can actually win internet arguments), but if the roc has been an option for a flying animal companion for rangers for so long, it begs the question on why Paizo would bother with such costly options like hippogriffs or griffons. They have released and rereleased (first online, then in an official document) the sabel company marine, and there is the monstrous mount set of feats.
There is little difference between a hippogrif and a large roc. Actually, there is a difference- rocs are far, far superior- less strength and dexterity, no special abilities on their talons, less natural armor (2vs8!), an inability to carry a rider until you get mastery (and even then the speed is cut in half). The last bit isn't there for sable company marines...but it still begs the question- if rangers could just get a perfectly functional flying mount from level 1, why would they ever bother with the hippogriff? Why would Paizo release this option three different times if it did not give you anything unique? Remember- Rocs ahve been around since 2010- some of these hippogrif bits have been released as late as 2014.
And why is a base ranger's options so much stronger than one that takes 2 feats to get?
So, when there is argument over the text, and a large body of following work by the designers shows rangers paying much, much more for a lesser choice while making it out as special... I cannot personally say I agree with this unless I see FAQs, erratas, or dev commentary.
if rangers could just get a perfectly functional flying mount from level 1, why would they ever bother with the hippogriff?
Standard rangers can't take flying mount from level 1, they gain the Hunter Bond at 4th level.
No, Rocs are not legal!
The ranger animal companion list is inclusive, there are no other options as it has not ever been officially expanded, with the exception of archetypes or feats. Your options are:
The second option is to form a close bond with an animal companion. A ranger who selects an animal companion can choose from the following list: badger, bird, camel, cat (small), dire rat, dog, horse, pony, snake (viper or constrictor), or wolf. If the campaign takes place wholly or partly in an aquatic environment, the ranger may choose a shark instead. This animal is a loyal companion that accompanies the ranger on his adventures as appropriate for its kind. A ranger's animal companion shares his favored enemy and favored terrain bonuses.
The flavor text in the Roc description is only flavor text. It does not actually allow you to take a Roc animal companion as a ranger. I believe it's left over flavor text from 3.5.
Remember, the core rule book and bestiary 1 had a lot of copy-pasta from 3.5 books.