I love tinned sardines as well, but I think the tins are the right size. No one else in my family will go near them and they do stink out the fridge and go bad quickly.
I agree with the above comments, they are cheap there is no reason to want to preserve them, there is just enough for two slices of toast and they dont keep fresh long.
If they came in kilo tins you would be throwing most of them away. Seems the right size tin to me.