Ken- The primary reason the seat tube has any angle is to allow the rider a certain amount of seat set back (behind a vertical line running through the bb center). This set back establishes the pedal/knee relationship. Common frame design thought has the seat tube angle varying with thigh length. Longer thighs "need" more set back equaling a shallower angle.
Another reason why smaller bikes often have a steeper angle (besides the thigh length being short) is to lessen toe clip overlap when short top tube lengths are used. (This goal is also why some small frames have slack head angles, pushing the axle further away from the crank).
There is a lot of marketing influence and inertia of tradition in making these design choices. In the ideal custom world the rider's dimensions and needs drive the choices.
An example of how it can be done is in the story of when I was working for Cyclery North in Chicago (1985). We were a frame building shop. The boss/designer would do the fit and design then hand off the actual build work to Tommy (or I). The boss based all his designs on a 60* angle between the down tube and the head tube (lower head angle). To make the more important frame dimensions work Tommy and I had to "fudge" this 60* spec. When I asked the boss about this he said "All the good handling bikes I've ridden use 60*". Sure if you're 5'10" and want no fender clearance!
Lastly why should the length of the stays or size of wheels change how the body needs to be positioned? I have four self built frames that share the main triangle dimensions but differ in the rest, as their use ranges from fixed gear track, through Sunday light, commuting to loaded touring.
> Date: Sat, 2 Feb 2008 08:48:50 -0500
> From: "Kenneth Freeman" <email@example.com>
> To: "'Emily O'Brien'" <firstname.lastname@example.org>,
> Subject: RE: [CR]Re: Classicrendezvous Digest, Vol 61, Issue 103
> Message-ID: <000101c865a2$5aa6fe60$6401a8c0@maincomputer>
> In-Reply-To: <email@example.com>
> References: <firstname.lastname@example.org>
> Content-Type: text/plain;charset="us-ascii"
> MIME-Version: 1.0
> Content-Transfer-Encoding: 7bit
> Precedence: list
> Message: 2
> Thinking about frame angles: I have two Italian or Italian style frames of
> the early '80s, with the same steep seat tube angle. My 1980 Masi and my
> '82 or '83 Mondonico are both 52/53 cm frames with 75 degree seat tube
> Is this a convention of the times? Is it a convention that is held today
> performance bikes? Is it just luck?
> The other dimensions, chainstay, top tube, front center, and head angle ,
> are rather different.
> Ken Freeman
> Ann Arbor, MI USA