Mozart too heavy

Well, yesterday I had a lesson with a student. That's when I noticed that she could play Mozart with almost all the necessary skills, spiccato, lifting, etc. But it just sounded heavy. It is a combination of both hands that contributed to that. Even at the loud places, the concerto No. 5 should not sound heavy. It was written in Mozart's early years, when he was without care for life's troubles. Even if Mozart were troubled in any ways, he took care not to reflect in his music. It was until much later in his life that he poured his soul out into the music. We would have seen the darker side of Mozart's emotions had he lived longer and write more.

But I digress. Something about how the bow as sustained on the string, and about the vibrato, that caused the tone to sound heavy, even on the high notes, especially when it got loud. Hard to explain in all the details of executing this "loud and light" tone. It just has to be done with the "final goal of tone color" in mind.