Scientific Reason for Salt Solution Gaining Volume
The vapour tension over the saturated table salt solution { the best  with the solid salt excess) is equivalent to the relative humidity about 75 %. 

If the rel. humidity is lower than 75%, the solution evaporates and the salt crystallizes.

If the rel. humidity is higher than 75%, the solution absorbs the air moisture and the salt is dissolving.

Such systems may be informally called "relative humidity buffers". They are useful for hygrometer calibration or for making a constant humidity aerial baths for long term weathering of devices or materials.

See the articles with tables of relative humidity above saturated solutions of various salts:


Researchgate.net
PDF on NIST  
PDF on Omega.com