Several factors. To my best recollection, from working with people of that generation:There's a lower mid range thickness/richness/density to things like a solo'd vocal stem or bass track - where they are clear but not bright or harsh in anyway. They just feel velvety, rubbery and massively big.
1. Technically. Using U67 and U47 microphones, which are amazing tube microphones. The signal then went through handmade mic preamps, those TG things that only exist at Abbey Road. Again, really great custom made circuitry, mated properly (impedance wise) to those microphones. then processed with equaalization and or compression before hitting analog tape. Engineers were trained in how to capture sound a certain way. This was adjsuted by the requirements of the beatles and George Martin, who had a particular sonic vision for these records. Sometrhing more than just the technology.
2. Musically. their arrangements indicated only certain numbers of instruments and voices. Today's productions, where you have people constantly recording TOO MANY THINGS believing this is the way to a "huge sound" is the main issue today. In ANY audio format, be it digital, analogue, whatever, you HAVE to know this fact: You can only put so much into a format before it is full. Think of your digital audio file as a CUP. One that can only ever hold so much quantity of sound. the trick here is to know how much of EACH sound has to go into the cup, in order to get to the best result sonically. THAT is the hardest part, and believe me, the arrangement is 90% of the solution to this problem, not anything technical or mathematical.
In the Beatles recordings, for example, when you hear backing vocals, how many do you hear? FOUR MAXIMUM for the most part, because that was how many members you had in the band singing the backing vocals. Later on, sure they invited friends to sing along on some of the productions, but what you notice on those, is that the backing vocals are less loud than when it was only 4 or less singers. Same with guitars: One for lead, one for rhythm. Not fucking 16 tracks of guitars or vocals. That never can get you that sort of MASSIVELLY BIG types of sounds that you like. Again, the CUP can only hold so much sound before it is full.
One guitar or voal will absolutely sound bigger than 16. Because it can occupy more of the cup space than 16. Those 16 have to be reduced in volume until they occupy the same space as that one big guitar.
Try this as an experiment: Record one rhythm guitar. Mix it down by itself so it fills your digital audio file. Do the same, but record 16 guitars, and try to fill the same level in your final audio file mixdown. You'll hear the difference quite clearly.