When describing something as "not big", there are plenty of synonyms to use to make your writing more dynamic. Some possibilities include "small", "little", "tiny", "minuscule", or "petite". You could also use terms that focus on relative size, such as "inconsequential", "modest", "limited", or "dwarfed". For a more poetic or descriptive tone, other synonyms could include "diminutive", "wee", "elfin", "miniature", or "pocket-sized". Ultimately, the synonym you choose will depend on the specific context in which you are writing and the tone you want to convey.