But why do you think there was more tax evasion in the Western part than in the Eastern part of the Roman empire ?
Should the WRE have taxed more its aristocracy ? Theoretically yes.
However it did not. So the question is : why did it not ?
It's not because the western elite had less civic morals than the eastern elite. They were no more no less virtuous.
It was because the human and financial burden to sustain all the challenges the West faced were too heavy for its resources.
This is why the WRE decided to downsize by renouncing military presence in Britain.
They lacked men able to serve among the citizenry. So they outsourced their defence to germanic auxiliaries. And they paid a huge price for it, being forced to give-up de facto control of huge territories of the WRE so that the germanic auxiliary could finance the military service they owed to the WRE. The last roman army by itself, the Comitatus, remained in Italy to ensure the personal security and the waning power of an emperor whose real area of control was dramatically shrinking.
The urbanization in the WRE was also dramatically dropping because the economy of the WRE could no longer support the cost of the roman urban way of life plus the cost of defending the WRE against the various threats it had to face.
Isaac Asimov has remarkably reproduced the process in the first or second book of his Foundation series.
I was basing my argument on Friell and Williams'
The Rome That Did Not Fall (which I'd advise everybody to read, BTW, since it's very interesting and really covers the period in detail). They come to the conclusion that the Western tax system was indeed less efficient, based largely on the recorded expenditures of the Western Senators. These people were clearly enormously wealthy, so clearly there was plenty of money in the Western Empire. The problem was raising it.
(On a similar note, Roman tax collectors were permitted to keep a certain portion of the money they gathered in lieu of payment. I forget the precise figures, but the amount kept by Western tax collectors was several multiples of that kept by Eastern ones, and this disparity only increased as time went on. Again, less efficient government and tax systems.)
In the West there was much greater polarisation of wealth (a few very rich magnates -- Gaul and Italy were pretty much entirely controlled by a dozen or so families, IIRC -- and lots of very poor people), so the wealthy could use their resources to get out of paying their fair share (bribing tax officials etc., who tended to come from the upper classes anyway and so had a vested interest in taxes not being too stringently kept), and nobody else was rich enough to contribute much. In the East, on the other hand, there was a larger "middle class", who didn't have the ability to avoid paying taxes but were still worth taxing.
Also, the Eastern aristocracy had largely been created by Constantine and his successors, and came from a region where royal cults had a long history. In the West, on the other hand, the Senate preceded the institution of the Empire, and a sort of sentimental republicanism persisted even down to this time, with a lot of Senators accepting rule by an Emperor rather than enthusiastically supporting it. Hence they were much less predisposed to make sacrifices at the Emperor's command than the Easterners were.
Manpower etc. did become a problem as the time period went on, but this was originally a result of the Western Empire's structural weaknesses (outlined above), although once established it did set up a sort of vicious cycle. On paper, the Western Empire c. 400 was more than a match for any of the Germanic tribes, and even at the end of the Empire it ought to have been at least as strong as any one of them. The West wasn't doomed to fall from the moment the Empire was divided; it fell because of bad government and a lack of investment on the part of its leading citizens.
(The above is my summary of what Friell and Williams argued. I'm doing it from memory, so if anyone has access to the book itself, feel free to correct me.)