@Jmjonesuk
For me fruit equals sugar which she has way to much of. I am not just looking at her BMI but she is visibly overweight with a large overhanging belly. I hear what you are saying about not mentioning her weight as this might give her a weight issue as she gets older but she already has a weight issue and it needs to be addressed by not only me but also my ex and my ex's family who I know over feed her and spoil her giving into all her demands. I feel like writing to the ex and just coming directly to the point about her and her family letting my daughters weight get out of control and risk the consequences of her stopping contact for the sake of my daughters health and mental health as she gets older.
Do you really want to give your ex the ammunition of "daddy says you're fat and horrible darling, but don't worry I love you"? Because if you write any such letter that's exactly how it will be presented to your little daughter. That will do wonders for her mental health
The fact you see all fruit as "just sugar" makes me think you are being far too rigid about this and about her diet in general. There are a lot of good things in fruit - water for one thing which most kids could do with a lot more of, vitamins and minerals, and dietary fibre which is very important for health. If you are fixating on her weight to the exclusion of her overall health, refusing her something as normal and necessary as fruit and frog-marching her everywhere so as to try and "undo the damage" of her lifestyle with your ex in your contact time, is be very surprised if she wasn't already aware you find her inadequate and are trying to change her - not to be healthier but to be thinner. So very damaging for a young girl.
Of course the best thing would be if you and ex could behave like adults and have a sensible conversation about how to have a loving, careful, united approach to getting your DD into a healthier lifestyle. As that's clearly not going to happen, then I would simply model the behaviour you want to see in your contact time ("in this house we don't eat sugary cereal, we have eggs/porridge/yoghurt and berries for breakfast"; in this house we take a morning walk to blow the cobwebs out"; "in this house we plan our meals and cook together, we don't get takeaways") and mind your own business about what happens outside of your control. Your ex won't listen to you, if you make a big fuss about your Dad's weight you will harm her emotional well-being (and a lot of weight problems stem from unmet emotional needs or damaged mental health taking refuge in comfort eating -speaking of what I know here!).
If what you want is a happy, healthy, well-adjusted girl then take the high road as suggested and learn to compromise. If what you want is a show pony daughter, then... Well...maybe reconsider that contact increase.