Yep, it may share the same data, but my point is that with more devices that "same data" will increase its size, don't you agree?
No, I don’t think necessarily I agree. You’re not creating more data, just because you have more devices. Whether you only have one iPhone and a Mac or an iPhone, an iPad and a Mac, has not by definition anything to do with the amount of data you need to store in iCloud to make the cloud syncing work. The only exception to this is device backups, but that is not for interoperability between devices.
iCloud Photo Library is a good example. When enabled, all your pictures will be uploaded. If you keep your devices in sync through iTunes, this means that your Mac will have the complete library. Your iPhones and iPads won’t add any more data by themselves, they just pull the data that is already in the cloud.
It may of course be true that you are more active when you have more devices, but that is a strictly subjective thing. Apple has nothing to gain by handing out free data on a per-device basis if it could equally offer a higher baseline and charge for higher tiers. This is especially true for Apple fans who own and have owned many devices.