I can confirm Razer Synapse is the cause--I uninstalled it, and Minecraft started up as it always used to, with my JVM arguments in the Minecraft launcher as -Xmx4G -Xms4G.
Before discovering Razer Synapse to be the problem however (thank you Minecraft help chat!), I managed to get Minecraft to start by manually adding the _JAVA_OPTIONS system environment value with -Xmx4G. (system restart required after setting this)
of course doing that will cause every Java program to use that option which is exactly what Razer is doing. Global options shouldn't be used to set parameters that aren't global, and how much heap to use is one of those non-global parameters.
Before discovering Razer Synapse to be the problem however (thank you Minecraft help chat!), I managed to get Minecraft to start by manually adding the _JAVA_OPTIONS system environment value with -Xmx4G. (system restart required after setting this)