So, I have only studied the medieval middle east so if you are studying a different time period this may not apply. Also, even within the Medieval time period, there were differences in leadership, as well as regional differences, that could also make the generalizations I am about to make inaccurate. However, once the Arabs conquered an area they would wage a poll tax, or jizya, on the Dhimmi peoples, meaning Christians or Jews (basically 'people of the book'). This tax was the same for everyone. One the other hand, the taxes that the Arabs paid were based on income. This means that the conquered rich had little incentive to convert, because they were paying the same amount as the poor, but the poor often desperately needed to pay lower taxes. In many cases, however, conquered peoples were not allowed to convert because the Arabs Muslims (being a relatively poor group at the outset of the Arab conquest that believed in egalitarianism) did not want to lose the extra money the jizya provided. So it is not accurate to say that everyone, or even most people, that converted to Islam were doing so simply to get a tax break. I hope that is a decent explanation. Please do not hesitate to ask questions, Middle Eastern history is so interesting I could talk about it forever--especially if/when I have more related materials at hand.

It should be first understood what the non-muslims and muslims were paying. Jizya was not the only tax that was paid by non-muslims, was an annual head tax set at flat rates and was paid by, as you stated, able-bodied Christian and Jewish males. The rates for Jizya was recorded to be 1 dinar for everyone but sometime exception were made for the modderate wealthy class who could pay up to 2 dinars and the wealthy who could pay up to 4 dinars, and sometimes harmless monks would pay as low as 12 dirham. Some early Islamic writings like Kitab al-Kharaj suggest that the tax was lower, and recorded it to be 48 dirhams for the richest, 24 for those of moderate wealth, and 12 for craftsmen and manual laborers. Also on top of Jizya there was a land tax called Kharaj which was similar to an income tax and was at a minimum 20% and maximum at 33%, although it could vary depending on the ruler.

The muslims on the other hand were only paying Zakat which was set at 2.5% and went to the benefit of Islam. Khums is not a land tax, it is a tax that warriors would pay after looting a treasury. When muslims would go off raiding, one fifth of the wealth accumulated would go to the state who would then distribute the wealth to charity and building mosques and institutions. When Abu Bakr first took control of Arabia he thought that maybe he could divert all their energy north(Rome and Persia) so the conflicts he just resolved wouldn't resurface and come back to bit him. Initially these were organised raids on cities which the arabs were doing prior to Islam, but later during Umar's the muslims discovered that the empires north of them were hollow from all their wars, so their wants went from booty to conquest. So the muslims had raided many treasuries khums became a big contributor since now 20% of it went to the benefit of Muslims, muslims quickly became the upper class in all these region that they conquered. So actually the muslim life style would appear as easier.

That basically covers where the empire initally got it's money, but over time when they were running out of places to raid, they had to settle in and begin ruling their territories, and many muslims didn't want to start paying a land tax so they made most of their money from taxing dhimmis. The muslims didn't evn want many of them to convert to Islam because then the converts would get to pay the smaller tax Zakat over Kharaj. Rulers would instruct the Arabs not to socialise and convert the peoples of the land. The mind set was "Go to church. Enjoy your Sunday. just pay your taxes". Even when people tried to convert to Islam some Caliphs would still charge them with dhimmi taxes.