But keep in mind that the Mediterranean was far more populated at this time, and wouldn't be again until the 1600s. There weren't as many Italians but far more EGyptians and Libyans and Phoenicians and Jews.
I am not sure what comparison you are making, but I certainly agree that most of the population of the Roman Empire lived fairly close to the coasts of the Med. You are also right in that the East was significantly more densely populated than the West, although Italy was likely an exception.
However, my understanding is that ethnic Jews were still a relatively tiny portion of all this Eastern population. It is not even clear from the sources that they were majority in the entirety of Palestine (although it is possible).
Palestine, in itself, is agains a relatively tiny portion of the Roman East, and not the most fertile. I don't think it could have had more than a couple million inhabitants, considering that Egypt (with a denser population and a larger overall arable area) had between seven and eight.
Of course, a lot of Jews lived outside Palestine, and had been for centuries before the Romans came to the area.
And we know that conversions happened, although I know no source that could provide anything resembling a reasonable number of those.
My guesstimate (not even approaching anything vaguely scientific) would be that in the first century AD, something like 5% Jews is reasonable (would be about three million, going by the debatable estimates of a total in the 60M range) but much more than that sounds excessive. Perhaps it was 10% indeed (sounds strange for a variety of reasons, though, I don't say it is
completely impossible) but in that case, it is hard to see all those people having proximate Palestinian ancestry, particularly as outside Palestine, the overwhelming majority would be living in cities.
(Which is part of why I think 10% total is way too much: the urban population of the Roman Empire is the most visible to us in the sources of all kinds, and if one Roman in ten was Jewish, we should expect a Jew every three or four people in cities; this is definitely NOT the picture we have from any epigraphic or historical source outside, of course, Palestine). Now, 10% of
urban population, instead... (that would be, dunno, perhaps 1% of the total one... but you have to consider the not insignificant rural Jews in Palestine, that of course, were for the most part ethnic Jews).