I have a Ribble Sportive Carbon road bike. The brake hoods are much smaller than most other bikes whihc makes them much less comfortable than other bikes (Scott, Specialised etc.) Can I just replace the hoods to make them more comfortable or put larger hoods over the top of my existing hoods?
yep they come in different sized based on what brand the gears are, and what year or model.
Campagnolo, Sram and Shimano the 3 main manufacturer's.
Looking at the stock Ribble sportive bike i'm guessing you will need these:
Campagnolo 10 speed, or modern 10 speed replacement hoods
Different makes and models of hoods are going to be different sizes, but as far as different sized rubber replacement hoods for a particular make and model of lever, I believe you are out of luck. The levers are designed with a particular hood shape in mind and I don't recall ever seeing a replacement that was of a different size.
You may benefit from changing the position and angle of your levers on your bar for extra comfort. Some folks like a deep dip in transition from their bars to their levers; personally I find it much more comfortable to have a flat transition from the bars to the levers, which can be achieved by rotating the bars back while pushing the levers down (which effecively keeps the levers at the same angle).
There used to be a velcro wrap that was padded and wrapped around the brake hoods. I believe I got it from Bike Nashbar or Performance shops. It made the brake hood "rest" area larger as well as helped to cushion road vibrations. I don't see them available any longer and could find no other types of replacements online, but I'd say that you could use some "stick to itself velcro that has the "loop" surface on one side of the band and the "hook" surface on the opposite side, so it will stick to itself when wrapped. You could buy longer bulk strips, say 12" long by 3/4" to 1" wide and wrap the brake hoods to the desired size. You may even be able to place some sort of padding on the hoods before wrapping if desired. This would be more cost effective to try before replacing the brake hoods with a different style.