My 5770 used to have 1gb memory and now it has 754mb?

Okay so I've never used this website before but I'm hoping to get some answers because it seems like the best to use.

Last February I purchased a new build with an AMD HIS Radeon 5770 which has 1gb video memory.
It had this up until maybe a couple of months ago.

In about October or November, my PSU completely blew and my computer wouldn't work at all.
So whilst still under warranty I took it back to the custom PC store and they fixed it, installed a new PSU and I was fine to go.

I cannot remember if I had 754mb's of video memory before that, however I checked a couple of days ago and I only have 754mb video memory now.
Which is quite frustrating to see whenever I use systemrequirementslab.com to see if I can run certain games.

So my question is has the blown PSU damaged my Video Card, or has it worn down over time or what?
Also can I return it back to 1gb?
Because I don't think overclocking the mhz of the cores will return the actual memory back.
I'm lost for answers and appreciate any feedback.