I'm starting to worry now

we got our two girls 4 weeks ago, they are 3.5 year old ex breeding bitches. They came with a large bag of puppy food

and when we asked how much they were fed we were told a "cup full" twice a day


After a lot of research I opted to swap them onto Burns, and have subsequently bought two 15kg bags of the chicken + rice kibble, which they seemed to be doing well on, they were weighed at the vets two weeks ago and their weight was :
13.6kg - kiwi
12.1kg - Lilly
However I was noticing sloppy poops in the afternoon, and on asking on here and Burns direct I found out I had been over feeding

as I was giving them 200g each per day
I reduced this gradually on the advice of Burns to 140-150g per day, but the Burns adviser did say "if they start losing weight they may need to go onto Burns Active"
Well I've just weighed them on the same scales and they have both lost quite a bit of weight:
12.75kg - kiwi
11.6kg - Lilly
I don't want to increase the amounts back up to 200g again as I was told it was too much, but I'm scared they are now underweight
I'm not sure what the expected weights are? I've just measured them, Kiwi is 16" high x 20" long (chest to base of tail), Lilly is 15" high x 19" long, both with a 22" girth under the armpits - is there an "average weight" for a working cocker of that size please?
I'm no expert but they do look a bit thin??





Should I swap them onto Burns Active? or is that a marketing ploy by Burns and do I drop Burns completely and try something else? If so what? there is so much information for and against every brand of food that I'm not sure what to try next??
They do have extra "high value" treats and chews, etc as well as their meals so shouldn't be dropping too much weight

They are also still very active and bouncy, although I noticed Lilly eating poo this week which I hadn't noticed before - another indication that Burns is not right for them
Help please??