Well I'm probably not the best person to answer that. First off, I have never noticed that at all with the brands Ive noticed as far as a better pump.
With the weight thing, depending on the brand Ive tried I gained from only a cpl of pounds to almost 10lbs... but like clockwork once I cycle off, I loose that weight post haist...thus why Im not a fan of it....if I personally dont see any sustained results(meaning keeping the weight after the product is done), I dont get better p[umps, strength or stamina gains...to me its just a temp fix to look 1% larger with muscle mass to back it up, and is just a reason to waste money.
But saying that, a lot of people are big proponents of the stuff and swear by it. As far as I know creatine is one of the most researched supplements on the market, with lots of supposed benifits...they just are ones I havent seen. But I'm usually a sceptic with most things. (Ive also never noticed any notable difference when using the majority of supplements branded to make you huge, while others that have taken it at the same time and same duration praise the products)
Personally, I think with a lot of things, any benifits may be the placebo effect...which may or may not be why you notice better pumps...you may tend to study your muscles more when taking it, and with the placebo effect maybe you are becasue your body and mind believes it. Which hey,...if it works it works...no matter how it works...(if that made sense...lol) It just doesnt work for me other than gaining a few useless pounds.
Someone else maybe able to give you a more 'scientific' answer...but thats all I got...lol