Can players weight change? If yes, when and how?

Yes. At training camp. Up/down in the direction of the average weight for the position.

They will also lose it gradually throughout the season.

So it basicly looses it effect as the season goes on?

Has this been confirmed? I couldn't remember if this was fact.

I've kept track of weight myself in player notes. It definitely does happen.

lellow, does the player losing weight as the season progresses affect his play ability?

And to clarify, are you saying ALL players loose weight as the season progresses?

All players will either lose or gain weight towards the average of their position throughout the season.

Yes, thank you Ares. That's what I thought was the case, but not sure.

My OTHER part of the question though, does the weight loss affect the players ability?

Yes - a heavier player will not be as fast, etc. Most of these are listed out in the attributes page. There are some others that I don't think are listed out like weight helps a player break a tackle (like strength helps a player drag a tackler which isn't listed either).

The players physical weight is somehow taken into account when determining the player's overall score, but I haven't seen the exact math.