Surprisingly this question doesn't have a definitive documented answer. Perhaps another data point would provide value to others looking for an answer. On my systems running CentOS (6.8,7.3) and Java 8 (build 1.8.0_60-b27, 64-Bit Server):
default memory is 1/4 of physical memory, not limited by 1GB.
Also, -XX:+PrintFlagsFinal
prints to STDERR so command to determine current default memory presented by others above should be tweaked to the following:
java -XX:+PrintFlagsFinal 2>&1 | grep MaxHeapSize
The following is returned on system with 64GB of physical RAM:
uintx MaxHeapSize := 16873684992 {product}