Sorry for a late response but wanted to give a scientific reasoning into why this occurs and why it’s normal.
You will pretty much always notice increase in strength but typically no gain in muscle mass in the first few months of gym. Essentially, when you contract your muscles, you are recruiting things called motor units, which is the amount of muscle fibres used for the contraction.
When you go to gym, you are basically tricking into your body that you’ll be constantly lifting these heavy weights in your day to day lives (which is why consistency is key so you keep up this stimulus). As a result, you have an increased neural drive to your agonist muscles (muscles needed for contraction). They have more recruitment of motor units, as well as a decrease neural drive to the antagonist muscles (your muscles that would do the opposite in that contraction). This neural change causes the increase in strength that you saw in your early months. This occurs before any growth of muscle.
Typically the next process would be loss of fat (making more defined muscles) and hypertrophy of the muscles (growth of muscles), but this occurs after potentially 6 months of gym or more (assuming consistency). It also depends on genetics, testosterone levels and other factors in determining your muscle mass. A big thing other people mentioned is diet, and that is also really important. Incorporating high protein is key, but don’t overdo it.
By the time you start noticing gains in muscle mass, your strength would’ve improved dramatically. It’s been 10 months since the post so hopefully you made some gains assuming you kept up the routine!
Ensure you are doing at least 65-80% of your 1RPM in exercises. This is the overload principle of strength training, and can help develop the strength further as well as hypertrophy. But keep up the routine, your large muscle mass is because your body THINKS you need that large muscle mass in your day to day life. Once you leave the gym and not go for a long while, that large muscle mass becomes a burden for the body (as it will need more fuel) and the body will just break it down to conserve muscle mass (which is why people who stay in hospital for a very long time and were stuck on the bed typically are thinner in muscles due to immobility)
Sorry for the long winded response but hope I was able to say something helpful and maybe even pass the message to any new future gym goers so they can understand the process of their body better!