There could be various reasons that there are less products in the feed then in the Magento Catalog.
To locate the reason of this, we've made a checklist.
Check if your products are enabled as only enabled products are shown in the feed.
Configurable / Simple products
If you've activated the option to export configurable/simple products they will be there in the feed. By default the Configurable itself is not exported as the configurable product by itself can't be bought. In this way the configurable products are missing which results a lower amount of products in your feed then in the catalog.
Filter Conditions: Visibility
If your using the configurable/simple export option it's recommended to select all the visibiltys in the multi-select box in the filter settings.
Filter Conditions: Category
Make sure you have the right congiuratiton on including and/or excluding categories.
Filter Conditions: Exclude Products on Stock level
If this setting is set to yes only products which are in stock are exported.
If there still seems to be a product missing make sure the product is visible in the front-end as well.