I have a decent all purpose saddle (english) from my last horse who was petite. Now we just got a new horse who is significantly larger. Do you think I should invest in a new saddle or just some really great saddle pads?
Theres some great videos posted to help learn about proper saddle fit, I'd do that, or have a fitter out and see if the saddle can fit the new horse. Padding up can just cause more problems in the end, and doesn't really fix a poor fitting saddle just masks the problem. Try checking your saddle yourself/having a professional do it then see if you need a new saddle maybe? Last thing ya want is back pain!
Saddle fit is very important. Especially in English saddles where there is relatively no padding between the saddle and the horse, with only the panels which can become misshaped or compacted.
Occasionally, you can improve the fit of a too wide saddle, perhaps if the horse has hollows behind the wither you can pad that out, but there is no saddle blanket that can fix a too narrow saddle. In fact, saddle blankets should do next to nothing for saddle fit at all in an English saddle.
However you horse being "petite" or not isn't really the difficult with saddle fit, it's going to be the angle and width of their back and withers. Depending on the tree width and the width of your horse then maybe it can be made to fit through restuffing, but you need a master saddler to do that.
It can be difficult to find a saddle to fit, what a lot of people do is try to contract the services of a saddle fitter who has a range of saddles to try on, who can come out and try a range of saddles on your horse to find out what fits best.