Your are right, you can't really compare it. But my experience is that regardless of the different volume data provided by the brokers, at least the overall performance is similar.
But you still can't really compare it 1 to 1.

Some authors say that the tick volume is proportional to the real trading volume.

I may only say that the volume range changes without an obvious reason. Probably when brokers change their liquidity providers, it reflects on the volume values.

I've a question about the volume data.
Since my own broker offers not much historical data,
I'm quite dependend on third party data feeds.

And during the search in finding a suitable data feed,
I stumbled upon the different volume information.

When you export the data via JForex you'll get the volume as total volume in millions or so (contract sizes), also Tickstory does this.

I thought, that in MT4 the volume shown, is just as ticks per bar?.

Does anyone know, which data makes more sense, the tick volume or the total volume?