Is being "a bit overweight" actually a medical diagnosis, or does it have to be more severe than that?

9), so yes, it does have a formal classification, it's just a step below the obesity threshold rather than not counting at all. Whether it becomes clinically relevant usually depends less on the label itself and more on whether it's associated with any actual health impact, which a doctor can help assess. References: https://obesitycanada.ca/healthcare-professionals/adult-clinical-practice-guideline/