Wavelet decomposition
This may be caused by "border effects" related to subsampling and signal length with perfect reconstruction filter banks.

For 2-band wavelets, there are standard  non-expansive solutions (for instance with periodic border extension) when $2^N$ divides the length $L$ of your signal. In your case, one or two levels work without length increase, becauses $20=20/2+20/2 = (10/2+10/2)+10$. Then, starting at level $3$, you cannot divide $5$ into equal parts, since $2^3 = 8$ does not divide 20.

So:


It signifies that you have decomposed too deep to keep the number of samples constant.
The increase happens because of your signal length, your decomposition  level and your border extention.


For some applications (outside compression) this is not really an issue.