I've shared a blog with tips to help you Increase efficiency going offline and syncing in Collector. It covers the Advanced Offline Options that let you, as the map author, specify if your users should download all features and their attachments, just features (without attachments), or nothing (no features) when going offline and syncing. This is just for the features you are retrieving from the server. No matter what you set in those options, the edits made in the field (features and attachments) are uploaded to the server on sync. Give the blog a read, then come here to discuss it.