I leave the straps on vests very long intentionally. This allows you to adjust the size to fit your pup and gives you more than enough room to allow for growth.
The girth straps I use are about 28" long. When you add that to the vest panel size, that gives the maximum girth that the vest can be adjusted to fit....
For example, a 20" vest would have a belly strap that can be extended so that the vest would clip around a 45-48" girth. The panel size would likely look small on a dog that large, but the vest would be wearable.
What do you do with the excess strapping?
The excess strapping can be secured up in the plastic components on the straps or it can be cut off and the cut end singed using fire. Burning the ends will seal the webbing so it does not unravel. I have seen people use all sorts of creative ways to secure the webbing.... zip ties, hair ties, string, velcro pieces, etc. I personally just weave the excess back into the hardware on the strap... but any way you like is perfect!