You have made enormous progress! I barely recognise you.
'Progressi' = pro-GRES-si, stress on second syllable. 'Enormi' = e-NOR-mi.
Use this when you notice significant improvement in someone's skill, fitness, Italian level, or any other measurable area. The phrase 'non ti riconosco più' (I barely recognise you) is a hyperbole that emphasises how dramatic the change is.
'Fare progressi' (to make progress) is the standard Italian expression. 'Enormi' (enormous, plural of enorme) agrees with 'progressi' (masculine plural). 'Non ti riconosco più' = 'I no longer recognise you' — used hyperbolically for dramatic positive change.
Sei migliorato/a in modo incredibile.
You have improved in an incredible way.
Focusing on the improvement process
In poco tempo hai fatto quello che ad altri vuole anni.
In a short time you have done what takes others years.
Speed of improvement as the impressive element
Il tuo impegno si vede chiaramente.
Your commitment is clearly visible.
Praising the effort behind the improvement
Italian culture celebrates visible improvement and the journey of self-improvement ('percorso di crescita'). The concept of 'migliorare ogni giorno' (improving every day) is a value taught from childhood through education, sport, and craft. Being told that your progress is visible is a particularly motivating form of praise.