The rear tyre can be replaced by a bigger tyre but it will affect the mileage of the bike and also the warranty of the bike will be void due to the modification to one of its part. The tyre size of the bike is no doubt visually quite small but provides a nice and required amount of grip to the rider. Even if willing to change it by going against the warranty, take car that the width of the tyre stays within the swingarm limits and do not change the positioning of swingarm in order to get a bigger tyre. You can safely go upto 100 section tyre as it will fit within the described limits.