Translation:The bathtub is too deep for the baby.
That vexed me too. However, troppo can be an adverb as well as an adjective, with the same meaning. So I believe in this case troppo is the adverb that modifies the adjective profonda. The adjective has to agree with the gender and number of the thing it modifies, which is vasca, but adverbs are invariant and don't change their endings, hence troppo, rather than troppa.