Fulfilment fee query

Hello
I’m fairly new to Amazon but I’ve just listed 2 lines, a medium and a large.
The medium sells for £20 and the large sells for £25 and weighs slightly more 200g v 140g.
I’ve been charged the 15% referral fee on both, which was 15% so all clear. However the fulfilment fee varies massively between the 2 and I don’t understand what is triggering such a high difference. (I wouldn’t have thought the weight difference would matter so much, dimensions identical)
So the fulfilment fees on the medium is £1.63 but on the large it is £4.06!!!
So on my large line altogether am paying £7.81 in fees which is 30% it just seems so high.
Has anyone had this before/shed any light. I’ve queried it with them but it’s been a week now and they are still looking at it
(Also is anyone else finding longer than normal reply times?)
Many Thanks in advance