What are the criteria for FBA small?

I have a few products that are small and sell well and want to send them to FBA, the first shipment has arrived with no problems and although the tracking still says it’s in Norfolk, most products are live on Amazon and Amazon has received everything. The small items have a low FBA fee which is fine and allows me to make a profit, so I am trying to buy the same products in different ‘flavours’ (everything else is the same, weight, brand and price) but although I even choose small in the prep menu, they are classified as normal and therefore the fees are more than the retail price. Before anyone asks, I am authorised to sell these products on Amazon, in writing by the licensed brand owner. I am not placing orders until I know things are going through ok but still, I see lost opportunities. Is there anything else I miss to enroll products in FBA small, the first time seemed to happen automatically. Thanks.