There needs to be enough groove angle to get good sidewall fusion in the narrowest part of the groove, then have enough groove width to manipulate the puddle and still get good fusion as the groove is filled, while minimizing the weld volume to keep weld time as low as practical.  In the root area, the bevel angle needs to be higher to have access all the way to the bottom of the groove.  This can be achieved with compound bevel (e.g. 37.5 deg/10 deg) or a J-prep with a smaller angle, but using the radius to open up the groove.  As the groove gets wider, access is better so the groove angle can be decreased while still having enough room to manipulate the puddle.  It's all about trade-offs to get the best results in the least amount of weld time.  In plate this is best achieved by a double vee or double bevel groove weld, which is not usually possible for circumferential welds in pipe.

Take a look at 'www.pipesaddlelayout.com; on the "PIPE SADDLE LAYOUT" page you can enter various pipe O.D. and I.D.; using a small angle such as a 20-degree intersection you can see the change in the length of the ordinate lines for different diameters.  Hope this helps.

DNC