Wattage indications should always be of the secondary side of the transformer.
However, the wattage always stays the same Etienne, wether you run a cab that is 100W at 110VAC or 230VAC, it _still_ consumers that wattage. The only thing that will change is the amount of current.
In a cab that is connected to 220VAC (let's use that number for easy calculation) that is 100W you will have a current of
P=UxI or Wattage=Voltage x Current
so the current will be
I=P/U, in this case: I=100W/110V= 0,91A
The same cab running at 220V will have a current of:
I=P/U, I=100/220 = 0,55A
So the current will be half of that what it will be at 110VAC.
This is also the advantage of using high(er) voltages, there will be less current, so the wires need to be less thick.
That is why the huge transportlines for mains run at 100.000V or higher....
That is ALSO why when you change f.i. an Atari cab from 110VAC to 220VAC (or 240) setting you should replace the fuse with one that is rated at about half of the original one !!!! This is often forgotten because the machine will work fine. When something goes seriously wrong in the cab the mains fuse will either not burn trough or much later, both can damage the electronics of course.
Bottomline: the amount of energy consumed by the cab stays the same.
Now....back to that transformer:
Yes that already looks a lot bigger than the one pictured on the site. This should be OK.
The Wattage ratings of cabs is usually also somewhat exaggerated (max. value) to be on the safe side.).
Most important: check how warm it gets after about an hour or so. IMHO if it stays acceptable (you can leave your hand on it for "forever") than it should be fine.
One note:
These cheap kind of transformers are all "autotransformers". I have already explained the disadvantage of those before here (
https://www.dragonslairfans.com/smfor/index.php?topic=1481.msg15436#msg15436)
There is a _very small_ risk that IF the isolation of the transformer will burn through for whatever reason, you will put 230VAC into the machine that expects 110VAC and you can figure out the damage that will happen.
However, true isolated transformers are way more expensive and are not found in this set-up (They are loose transformers).
