depending on how you have your ipod attached, just changing the volume settings on the ipod may or may not do anything. if you are running a wire from your output to the aux in on your headunit, then yes, you can adjust your volume through the ipod and the head unit. (you are still on the 2012 correct?). but if you have it connected via the ipod controller, then changing the volume on your ipod does nothing for sound output.
the volume of the ipod should be roughly equivalent to the other input sources. if it is not, then the first place I would check is the quality of your mp3 that you are trying to play. 2 things will come into play here, the bitrate of the song, and the volume at which it was created. some older songs are a lot lower volume than the newer stuff.
my entire music collection will lnot fit on the ipod. so what I do is to periodically shift a bunch of songs over into a directory (I call this 'tunes'). I then run a program called mp3gain on that directory to 'equalize' the output of all the songs so that they are roughly equal. be careful with this program, you can introduce a lot of distortion and clipping if you go too high. I stick with the default of 89.5 and I get plenty of volume on the highway. I also have ripped all my cd collection at 320kbps using the program 'exact audio copy', and I use the website mp3va.com to purchase any mp3s I want. they often have sales where tracks are .10 and if you buy the entire album, you get a 20% discount. plus if you add a $30 credit, they will match with $15 bonus. I have not had any problem with 'unauthorized purchases' using my cc, but if that were a concern, you could always get one of those prepaid cards from Walmart and use that. I then delete everything on my ipod and then import this directory into iTunes (I do not keep my entire library in iTunes either) and then upload that to my ipod.
I've also imported this directory into my garmin so if i'm using gps, I can still listen to my music via it's media player and still get turn by turn directions. now, the garmin media player does have a considerably higher volume than all my other input sources, so if i'm using that, I do have to turn the headunit down 3 or 4 bars.
hope some of this helps, and if anyone else is reading this that has a 14 or newer, skip the ipod part (imo). just use a thumbdrive, much easier, and don't have to mess with iTunes. mp3gain, and exact audio copy will still work for you though.