from space and to space size is different and varies

[Thomas Rohde]

Hi Folks,

up to yesterday I always thought, that from-space and to-space have 
always the same size. In a GC log of a colleague I saw the following and 
was wondering about it:

22.10.2012 00:01:59 {Heap before gc invocations=590:
22.10.2012 00:01:59  PSYoungGen      total 72768K, used 70928K 
[0xcdc00000, 0xd3800000, 0xf8800000)
22.10.2012 00:01:59   eden space 54528K, 100% used 
[0xcdc00000,0xd1140000,0xd1140000)
22.10.2012 00:01:59   from space 18240K, 89% used 
[0xd1460000,0xd2464168,0xd2630000)
22.10.2012 00:01:59   to   space 18048K, 0% used 
[0xd2660000,0xd2660000,0xd3800000)
22.10.2012 00:01:59  PSOldGen        total 1400832K, used 334889K 
[0x78400000, 0xcdc00000, 0xcdc00000)
22.10.2012 00:01:59   object space 1400832K, 23% used 
[0x78400000,0x8cb0a778,0xcdc00000)
22.10.2012 00:01:59  PSPermGen       total 20480K, used 19997K 
[0x74400000, 0x75800000, 0x78400000)
22.10.2012 00:01:59   object space 20480K, 97% used 
[0x74400000,0x75787568,0x75800000)
22.10.2012 00:01:59 299113.923: [GC [PSYoungGen: 70928K->15357K(72512K)] 
405818K->350331K(1473344K), 0.0717791 secs]
22.10.2012 00:01:59 Heap after gc invocations=590:
22.10.2012 00:01:59  PSYoungGen      total 72512K, used 15357K 
[0xcdc00000, 0xd3800000, 0xf8800000)
22.10.2012 00:01:59   eden space 54464K, 0% used 
[0xcdc00000,0xcdc00000,0xd1130000)
22.10.2012 00:01:59   from space 18048K, 85% used 
[0xd2660000,0xd355f7d8,0xd3800000)
22.10.2012 00:01:59   to   space 18112K, 0% used 
[0xd14a0000,0xd14a0000,0xd2650000)
22.10.2012 00:01:59  PSOldGen        total 1400832K, used 334973K 
[0x78400000, 0xcdc00000, 0xcdc00000)
22.10.2012 00:01:59   object space 1400832K, 23% used 
[0x78400000,0x8cb1f778,0xcdc00000)
22.10.2012 00:01:59  PSPermGen       total 20480K, used 19997K 
[0x74400000, 0x75800000, 0x78400000)
22.10.2012 00:01:59   object space 20480K, 97% used 
[0x74400000,0x75787568,0x75800000)
22.10.2012 00:01:59 }

Before GC from space has 18240K and to space has 18048K.
After GC from space has 18048K and to space has 18112K.

java version "1.5.0_20"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_20-b02)
Java HotSpot(TM) Server VM (build 1.5.0_20-b02, mixed mode)

1. Why is the size of from space and to space not equal?
2. Why is the size always changing?

Bye,
Thomas