The three reasons you state before saying not too buy are the three reasons you should research items before you buy.
Its you own fault you did not look at the size, weight and grip style. Had you first looked for help here on OCN you would have been guided to a mouse more suitable for your grip style and with a weight closer to what you require.
Theres lots of help available here 24/7, theres always heaps of people online willing to help. So make a note that before you buy again...seek advice upfront to avoid being dissapointed.
Also for the record, yes i do own a G400 and its a great mouse for me and im happy to defend it, its so damn accurate and it fits my grip style well,that said i do have my gripes about where my little finger rests and the left button has gone shiny
but these things are small and when you see how cheap the mouse is to buy...its easy to put these issue to one side.
Hope that helps.