Well yes, it would need to be huge, but I think the whole point of that sort of thing is the 'room sound' - not just directly micing up each performer, but also having wide range mics picking up the broader ambiance. Kinda like why drums are sometimes recorded in stairwells and funny places (the reverberations don't bleed off into a large room) - that's less-relevant to an orchestra, but recording engineers like to get the organic room sound as well so where appropriate they can mix it in