I wonder if that's because the heat drives off some of the moisture (because there's usually quite a bit of water tied up in sodium carbonate crystals sold as washing soda)?
The answer to the question is not entirely simple because in part it depends on the temperature of the solution, but I found
this page that gives the solubility for sodium carbonate decahydrate (which is what washing soda is, at least as far as I'm aware). I doubt the solubility is linear with temperature but they don't give enough of a range of temperatures to attempt a graph. I'd guess if the solution were kept in a house where one might not expect temperature to drop below about 15°C then maybe 150g to 160g per litre of water is the highest you're likely to be able to go. So if you have 25kg you're going to need a fair bit of storage
We buy cleaning products such as washing up liquid and detergent for the washing machine in 20 litre containers and I was contemplating using them for a "bulk store", but I reckon eight would be required for 25kg of crystals.
Vacuum sealing does sound like it might be an easier, err, solution
If it still goes solid just do as it says in the article about heat: cut the bag open, dump it in the correct volume of water and wait for it to dissolve.
James