Yes, you'll be fine. I flew Easyjet last week on the el cheapo fare, so was only allowed the small bag. I used a coolbag which measured about 45 x 40 x 20, so would have been too tall if I'd filled it but I didn't so it would have squished down. In practice, no-one checked even though it was a packed flight - so long as it's not very obviously too big, or on wheels, then they don't seem to look too closely..