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This is a good point - it needs to fit. Part of the reason I got mine at decathlon was so I could try them on (if you do this, make sure you're wearing what you would wear for running - this applies more to people with boobs I suppose).
hats-
I had a look at the Camelbak sizing and it says one size fits most but I've got a 37" chest and have it adjusted almost to the minimum so i would say much bigger than a 40" chest could struggle definitely no bigger than a 42 but some of Decathlons new packs come in 3 sizes and the top of the line one looks packed with features and is only 40 quid so has to be worth trying on at that price. I've used a really expensive Salomon bladderless vest and whilst being extremely light and comfortable in the heat the durability of the material was terrible and even catching it on brambles and twigs ripped it to bits.
Retro_Bastard
Have a look around online if your looking for anything Camelbak as 20-30% discount seems to fairly standard but still a little more expensive than Decathlons prices which are generally unbeatable for their price-performance across most of their lines. Check sizing for running packs as well as it'll drive you mad if it swings about or rubs anyway. Many come in at least 2 different chest sizes.