It totally depends on the conditions it was made under. In theory anything between 15 and 25kg. For small bales there isnt really any standard as so much depends on water content and whim of the person baling it!
It depends on how they are baled - they are all different really. The best option is to get a set of scales that you hang a haynet on and weigh your hay out in a net. They aren't very expensive.....
have one of those scale things - and have just had instructions from the nutritionist to increase his hay to 8kg a day!! was wondering how much a bale weighs as I'm sure this is going to cost me an absolute fortune!!
I would not get too hung up about weights of bales of hay really...he is obviously hungry if as you say he is dragging you about everywhere for food...perhaps up it by a pad and see how it goes, keep weigh taping him etc. I would say that loosing 6kg a week is a bit much really...I prefer them to loose it a bit more gradual than that.
My boy (Welshie cob) is on a diet and weighs 510kg at the moment...whilst increasing his exercise and having him in on hay at night I aim for him to get down to his ideal weight of 490kg over the next couple of months so he is slim for spring time. He is out at grass during the day (plenty of it) and in on ad lib year old hay at night.
Just keep up the weigh taping and exercise, but there is no point making them go for long periods without food as they just get grumpy and difficult (or is that just mine?)!
I've had to increase haylage amount this winter because of the (extremely) poor summer we had in Tewkesbury there is hardly any nutrition in it. I'm feeding nearly twice what they were on last year plus additives etc and they aren't putting on weight.