I had a betta for many years and I agree with the people above. The betta will not get bigger than this and needs way more swimming space. The water conditions are probably really poor and the fish is most likely experiencing some form of stress from the lack of mental stimulation. I recommend a larger tank with plants (plastic will do) and gravel. Also a filter would be helpful as well.
If a tank is currently not available to you then you might want to consider using a clear, cleaned plastic container as a temporary tank. You can use it until you get a tank and it works pretty well. The only negative being that you can't observe your pet as well Also very important if making the plastic tank either live a few inches so the betta can't leap out or put a lid with may holes so the betta can breathe.
A bit like this
I didin't produce any of these photose or own them in any way or form
What is the volume of that tank when it's filled up? bettas need an absolute minimum of 2.5 gallons/10 litres with 5 gallons/20 litres being a lot better. That tank looks smaller than 2.5 gallons in the photo - and it needs filling to almost the top.