When you begin your fitness program, especially after not having done anything for some time, your initial strength gains will typically be quick and dramatic.
During your first 12 weeks, it’s not uncommon to witness a 10-30% increase in overall strength. The strength gains during the initial weeks of your new training regimen will derive from the recruitment of new muscle fibers that make the muscles more visible and stronger.
While muscle recruitment does not lead to increased muscle mass, it definitely helps in making the muscles appear bigger. One of the explanations for this is the fact that muscles take in water and swell during training. Another is that muscles tend to burn fat, which then makes them more prominent in appearance.