This may have been based on a comment I made about the canard that "Islam was spread by the sword." That phrase does not refer to the relatively straightforward notion that Islam had an easier time spreading in lands taken during the Arab conquests and thereafter under Islamic rule, but rather the very specific charge that non-Muslims in these territories were given the option to convert, pay the jizya or die, and thus that Islam was primarily spread through forced conversions.

That is simply untrue. The conversion process appears in most instances to have lasted hundreds of years. Forced conversions were rare. The jizya was not initially applied in a punitive or discriminatory fashion (and in fact taxes had to be created for Muslims in order to account for the lost revenue, and conversion did not in practice it seems actually exempt one from paying the Jizya).

This is not a speciality of mine, but to areas where Islam spread that were not conquered we can also add sub-Saharan Africa and the spread of Islam into China, which was extensive enough that Ibn Battuta was encountering mosques in Fujian during the Yuan dynasty.

As for why the initial spread of Christianity was different, the concept of state religious orthodoxy didn't really exist in the antique world. It's a concept that arises out of Christian states in a way that boxes out competitors. Christian states in late antiquity were intolerant of dissent within Christianity, let alone of a new faith. In other words Christendom at various points has tried to violently impose orthodoxy on itself, whether or not some form of Christianity may have spread non-violently or without state compulsion prior to that.

Moreover, unlike Christianity, Islam is deeply connected to the concept of a government or state. Muhammad was not merely a prophet, he also ruled territory, in stark contrast to the lifestory of Jesus. So it's unsurprising that, in Weber-ian sense, Islam gained a connection with conquest and state violence because that's precisely what a state is.