Combo of both. 45% of 225 is more than 40% of 205.
Either way check what clearance you have now from the struts as mine comes pretty close of 205s. 225s may rub on the strut / inner guards on full lock.
Over the 205/40 for example, you have gone to a bigger diameter and a wider width, so you're just increasing the chances of getting rubbing. That's ignoring any handling difference having the wider tyres may or may not having on the car.
We'll ignore the fact that if a WOF inspector was picky you could fail for using tyres greater than 5% bigger than factory.
