The final tooth proportions will affect how much of the gap should be closed. This is essential to creating an aesthetic smile.
As the teeth get wider (when the gap is closed), short teeth can look very squat and squarish. This can be corrected with gum surgery to remove excess gum prior to the composite bonding.
Orthodontics can close gaps without needing composite bonding.
Also, there is the issue of hygiene. By closing a very large gap, an undercut is created at the base of the tooth. This is prone to trapping food and can cause gingivitis (swelling and bleeding of the gum) in that area.
In back teeth, closing a very large gap in between the teeth can cause the composite material to fracture because the material is unsupported by underlying tooth structure. Also if the tooth is loaded off-axis due to pre-existing tipping/tilting, this may cause the tooth to drift further.
