The size of a speaker means nothing. The sound depends of something like a dozen different parameters and the box.
Also depends on the volume. A speaker being driven at 50% will sound better than the same speaker being driven at 100% and a cheap speaker won't sound as good as a well made, more expensive, speaker no matter what size it is. IMO even with a quiet jazz drummer a 10" will sound better than an 8" speaker.
You're happily using a 25watt combo, so I'd suggest you get another combo, the same as your current one.