My lemons are gigantic with think skin. They take forever to ripen and by the time there even bigger.

Think rind of citrus is generally an indication of imbalanced nutrients, either too much nitrogen of not enough potassium. You may try weekly application with 'Yates Thrive Citrus & Fruit Natural Fish & Seaweed+' as a complete plant food, specially developed for boosting healthy grown and yield of juicy fruit in citrus and other fruiting plants. 

Hope this helps and happy gardening :)