the inefficiency of noncontiguous data - why?

I noticed that breaking up a large array into 2 pieces results in a much higher memory usage for my program. Example:

A program with:
float[] a = new float[32000000];
uses 138MB of memory on my system.

But a program with:
float[] b = new float[16000000];
float[] c = new float[16000000];
uses 187MB of memory, even though it's the same amount of data.

Why is this? My heap size is 800MB.
.

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