The size of an item depends on its importance value: More important items are bigger, whereas less important items are displayed as smaller thumbnails. There are quite many factors affecting the importance value of an item incl. its rating, the fact whether it has been renamed or tagged, the number of comments and likes it has etc.
In short, there is no direct way of controlling an item’s size. However, you can increase an item’s importance in various ways. For example, chosing a “five star” rating usually does the trick – unless there is another “five star” rated item nearby.