Both sets of leaves look very rhododendron-like so I would completely discount the possibility of another genus. I also would not make anything of lighter color and thinner texture at this point in the year. All younger rhododendron leaves are lighter in color and thinner in texture until enough time has passed for them to harden off. I would also account for the differences you observed last year to differing fertilizer and general care regimes. The larger deeper green leaves were likely left from the time the plant was cared for properly while the newer ones probably received indifferent care resulting in stunted growth and paler color, not to mention the plant's overall beat up appearance. Branch sports are extremely rare in rhododendrons so chances are you have a single plant of r. maximum.

Thanks so much for your thoughts! So should I just let it grow for a few seasons and see if the stems with the smaller leaves start looking more like the large leaves? (In case it wasn't clear from my first post, all the smaller leaves are on completely different stems than the large leaves. It's not a case of having large leaves below with the smaller, possibly stunted leaves on top, or even on separate branches from the same stems. The stems with large leaves have leaf buds but they haven't opened yet.) Thanks!!