Java has always has the largest population for at least the last 2 millenia since its land is dominated by vulcanic arable soil. Sumatra has too much peat land and was historically rather thinner then present day. There's a reason why Srivijaya was a trade-ports-focused thallasocracy. Though you might want to point out the Cham case, where they had been sitting on what the Vietnamese will later view as future lebensraum for centuries without actually developing extensive agriculture when they should've been able to, or Malay Peninsula which until recently was sparsely populated. But there's no doubt they were seriously influenced by Srivijayan political model.
What I would regard as serious potential for additional agrarian central would be the area covered by South and Central Kalimantan provinces. Kalimantan is largely ancient jungles covering mountainous terrains when it's not peat land and swamp but there are some arable river valleys such as that as well as Kutai valley in East Kalimantan, if smaller. Had Dayak Besar actually developed into extensive agricultural society early enough, even on just Bugis level at the beginning it won't just cease to be Javanese natural lebensraum target, they'll might become to Java a menace a la Japan was to China.
Another potential is obviously Papua. But you gotta cheat with some mutations a la Jared's LoRaG to pull this off. And also you can't have Austronesian settlement to start it with since Austronesians came from the north, so the Highlands and Asmat lower plains are way beyond their reach.
There's no way other island can have Javanese population density. Central Kalimantan plains alone might come rather close, but it's just a fifth of the whole island. Papuan soil isn't very good for rice cultivation without modern technology, and as mentioned it's not like the best parts are within the reach of ancient Austronesians. I imagine Papua could've multiplied its population by 5 to 7 times at most. Approximately 50 to 70 millions. A lot stronger then OTL. But it doesn't come nearly anywhere near Javanese population density.