The most famous case, perhaps, is La Mer by Debussy.
And there are more sinfoniettas that could have done without the diminutive. I'm thinking of E.J. Moeran's Sinfonietta, rather his Second Symphony.
Another case is Bohuslav Martinů. Some of his later orchestral output could easily be regarded symphonies, natural successors to his Sixth, Fantaisies symphoniques. In particular his Frescoes of Piero della Francesca ('Symphony No. 7'), The Parables ('Symphony No. 8') and Estampes ('Symphony No. 9' - or a higher number, taking into account other works liks the Toccata e Due Canzoni as well.
And there are more sinfoniettas that could have done without the diminutive. I'm thinking of E.J. Moeran's Sinfonietta, rather his Second Symphony.
Another case is Bohuslav Martinů. Some of his later orchestral output could easily be regarded symphonies, natural successors to his Sixth, Fantaisies symphoniques. In particular his Frescoes of Piero della Francesca ('Symphony No. 7'), The Parables ('Symphony No. 8') and Estampes ('Symphony No. 9' - or a higher number, taking into account other works liks the Toccata e Due Canzoni as well.