Hmm, my renders are generally pretty big anyway... another possibilty.. you should have an oportunity to set the compression quality as well... I think this pops up when you save... the default is 75, use 'no compression' or better yet, switch to .tif as opposed to .jpg. The tif filetype also carries an alpha channel with it (when rendered over background color)which is extremely helpful when layering renders in PSP or whatever. I generally get as good or better a resolution as I can use. Maybe someone more Mac familiar can help.