I tend to agree with @TheRubberDuck. It is all in the nuance, and the verb tense, of the spell as written:
All equipment worn or carried by a creature is similarly enlarged by
So, all equipment worn or carried by a creature at the time of casting is similarly enlarged by the spell. The magic has been cast at this point, there is no new casting of the spell happening, or transference of the spell as a character roams around and picks things (or people) up. Also note:
Any enlarged item that leaves an enlarged creature’s possession (including a projectile or thrown weapon) instantly returns to its normal size.
So if your target character/creature holds onto another, secondary character/creature while he is being Enlarged the spell could arguably affect the secondary creature, but only while being held by the target of the spell.
I would argue the same for Invisibility, for instance. If a half-orc let a halfing ride on his shoulders and was made invisible, they'd both be invisible, but once the halfling left the half-orc they would visible again. The wording in Invisibility is a little clearer, emphasis my own:
Items dropped or put down by an invisible creature become visible; items picked up disappear if tucked into the clothing or pouches worn by the creature.
So, when an invisible creature picks something up and hides it on their person it doesn't become invisible itself, it becomes invisible because of it's circumstance hiddin within and invisible creature's garments. With regards to Enlarge, I'd apply the same logic--the spell doesn't pass on to something picked up.