Load Minimum Kakwa

Hi all. Thinking about getting a Kakwa either 40 or 55. I expect for the majority of my trips. Total weight will be around 8kg to 13kg including water and food, and that the 40L will generally be big enough. That said, I am minded to go for the 55 for the extra "what if" capacity. Is there a minimum load in these bags? If I pack 8kg in the 55L will it be uncomfortable due to lack of weight for example? Thanks!