In my humble opinion, this is really the conductor's problem. While overall interpretation of the work should be a shared endeavour between the soloist and the conductor (which I recognise can be quite a problem, given that the egos of the average soloist and the average conductor are considerable!)(and is even worse if there is a significant difference in experience or age) it is the conductor's responsibility, within that shared interpretation, to ensure that there is the desired balance between all the instruments involved -- the orchestral instruments as well as the soloist.
This balance problem needs to be resolved in rehearsal -- which must be done in the hall where the performance is to be played.
The soloist does have a responsibility here too, though -- that is to play the piece the same way during the rehearsal as he or she does during the performance. The conductor will not appreciate surprises...