It doesn't take much "wattage" to heat a small object. Even some small mains powered soldering irons are only 18 watts and think how hot that gets. Think how hot a small filament bulb gets... it's all the same.
I would say it's perfectly normal if you had an extended listening session at high volume. A lot of energy is in the upper bass/mid range and 99.7% or so goes as heat in the voice coil.
And forgeting your speakers for a moment... it's your ears might begin to regret it later on.