Using these comparison photos for reference of this article.
Meet Mummy Ida and her 2 toddlers who are at 2 years apart. It may seem both of them can fit into a same size carrier. However, if you look at the pictures closely, her younger son, Zafri (left side of the picture), sits better in a Basic Size 1 (Kiro/green). Zafri’s thighs was able to rest comfortably with knees out of the carrier. The panel supports his back snugly right up to his neck. Compared to when he sits in the Basic Size 2 (Tano/blue), he seems to sink into the carrier a little and with the leg out padding pushing his calf away. Ideally, his lower legs should rest downwards at 90 degree angle at the knees. This is a sign that the width of the carrier base could be a little too big for Zafri.
Ziqri, who is 3 years and 7 months in right side of the picture, may appear to fit in both Basic Size 1 and Basic Size 2 alright. Pay attention to his knees. Ziqri has better “m” shape position when seated in the Basic Size 2 carrier. You can see clearly his knees are much higher in the toddler size. With knees higher, most of his weight is rested on his bottom, rather than his legs. This makes carrying much bearable as there is lesser gravity pull coming from his feet as it sways.
Note that this is just age and weight reference. Fitting in carrier is still subjective to each baby’s development.