Why do S0C and S1C fields keep changing in the output of jstat?

Question

Here is an output of jstat:

jstat -gc 7285 1s 125
 S0C      S1C      S0U      S1U      EC       EU        OC         OU    ...
22016.0  9216.0   0.0     9120.0  156160.0 153482.7   125952.0   31227.0 ...
13312.0  20992.0  9472.0    0.0   151040.0   2975.5   125952.0   31227.0 ...
13312.0  20992.0  9472.0    0.0   151040.0   5951.9   125952.0   31227.0 ...

As per the oracle documentation:

S0C Current survivor space 0 capacity (KB).
S1C Current survivor space 1 capacity (KB).

Isn't the capacity supposed to be fixed (i.e. S0C and S1C fields)?
Why are those fields changing with time?