The fullness of the sound is mostly determined by the size of the guitar, its constructions and the materials used in its construction. You cannot change any of those. You can make the sound worse by paining the guitar, adding stickers or using old strings, so don't do any of those things. The one thing you can control is the set of strings you use. Light gauge (thin) strings are easier to play but have a thinner sound that does not project as well as heavier gauge strings. Adding heavier gauge strings will give you a fuller sound but will be harder to play (the fingers holding down the strings have to work harder and may get sore for a while). On some guitars, the extra tension of heavier gauge strings can cause the neck to flex a bit, which will tend to make the guitar harder to play.
If you have access to some amplification, an inexpensive 'sound hole pick up' will enable you to amplify your guitar to get a much fuller sound. They are cheaper than a new guitar but expect to pay around £50/$60 for anything good enough.
I am sorry but there is not an easy solution to your question. You already know what the real answer is - a new full sized guitar - but that may not be an option. That's OK, most of us are not rolling in money and are happy to make the best of whatever instrument we've got. If a new guitar ever becomes an option, please post another question so we can suggest what key features you should be looking for.
EDIT: The comments about using a bone bridge saddle and nut are not wrong but even many experienced players would be unlikely to hear the difference and, unless the construction of the sound board is of high enough quality (solid, mature tone wood - typically spruce or cedar), it would not make an audible difference. I am unconvinced about the effect of bridge pins.
Guitar player for 50+ years