(Not gargantuan orchestras, of course, just the same size that would be considered normal for early 19th-century repertoire for the orchestra and the venue in question.)
I really cannot understand the continued preference for reduced strings in performances of Mozart symphonies by modern orchestras in modern halls. I’m a huge admirer of the period instrument movement and of historically informed performance generally, but this practice is one of the less desirable side-effects. It does seem as if a little knowledge is a dangerous thing, or at least leads to a misguided vision of a fictional “lean, classical sound”.
Unfortunately, unless you have something like a perfect acoustic (can we move Severance Hall to Bennelong Point please?) reducing strings in 18th-century repertoire seldom really works. By which I mean it doesn’t achieve anything like the effect or intention the composer was driving at.
We know from Mozart’s letters that he liked big orchestras. Although there would certainly have been performances with smaller bands, in Vienna his symphonies and concertos were sometimes performed with 80 or more players (i.e. 60-something strings) and there’s a documented performance in Vienna in 1781 (Robin Stowell thinks it was of the “Paris” Symphony in D, K297) with 40 violins, 10 violas, 8 cellos, 10 double basses and doubled woodwinds (including 6 bassoons!). And it was in Paris that Mozart wrote home to report his excitement at the unusually large forces he had at his disposal.
Of course, 30 baroque violins is not the same animal as 30 modern violins – I know that. The higher tension, steel strings and modern bows make a world of difference; playing techniques have changed. So the fear of unwieldy heaviness is a natural one. But going to the opposite extreme isn’t necessarily a solution. Better to get a good string section of 50 or 60 playing with an articulate and vigorous sound than to reduce the numbers and remove the backbone of the music. [Chamber orchestras may want to raise an objection to this view, but such ensembles are accustomed to projecting the necessary sound with their smaller numbers and are not unknown to use electronic enhancement when the acoustic makes it necessary.]
Mozart should be more than pretty, and this is one instance where I wish we could return to the “bad old days” and hear more full-blooded Mozart being played by full-size orchestras.