I am just creating a little program to design spells with your new Ars Magica book when something I have thought about many times over the years came up.
Beast of Outlandish Size is a MuAn15 spell. Base 4.
What if I create The Big Pig? This does the same thing, but whereas BoOS effects any kind of beast, The Big Pig only works on, well you guessed it, pigs. Should restricted versions be a magnitude lower, or can Hermetic Magic not be specialised like this?
The level should remain the same.
There is usually no benefit in making a spell more restrictive than the guideline used requires.
"Restricting" a spell to something narrower than what the guidelines and examples allow does not lower its level - among other things, this would unbalance spontaneous magic: restricting a spontaneous spell to "this one target, this one time" is no restriction at all!
But "restricting" spells can have other benefits, to the point that it's not even clear it can always be done. For example, a spell to turn a pig, and only a pig, purple would tell you whether the target is a pig, even when it would not be otherwise apparent, even without using Intellego. This has been discussed at some length here.
That is an excellent idea! A spell that makes only werewolves turn purple will be much lower magnitude than a spell to detect werewolves! 
Restricting a spell or enchanted effect can be useful in a lot of situations. The most common is if you only want to affect one thing in a mixed environment.
Sure but there are multiple types of wolf shape shifters who would not be affected by your spell targeted at "werewolves".
The question that arises is how the magic knows that x is y. I mean maybe a spell that turns pigs purple works on anything with the essential nature of pig; but what about a spell thst works on Structure (court) and makes murderers turn purple or adulterers turn scarlet? The restriction has now become a plus but murderer or adulterer would not be essential nature? I would think at the very least the spell would need an Intellego requisite if it could work at all?
Generally speaking, the main benefit to restricting one's spells is that they fall under a Magical Focus or Potent Magic that you possess.
Say your Magus has a mMF in Fish. You might want to research a powerful spell, and could do it faster if you restricted it to just fish, and not other animals, so that then your mMF will affect your Lab Total. If you hadn't restricted it, your Lab Total would be lower, so the spell might take longer to research, even if you'll gain the mMF benefit when you cast it only on fish.
Another purpose to more defined spells is often in the form of bonuses, shape and material generally by faery sympathies and charms can also be a significant factor.