The OpenLogicalChannel indicates in dataType.audioData the MAXIMUM payload size for the forward channel to be opened. In non-fast start, the endpoint opening the channel would obviously use the information it gathered in the terminal capability exchange in order not to try to open a channel that would exceed's the receiver's capabilities.

In fastStart, there is no terminal capability negotiation. The endpoint sending the SETUP message includes the MAXIMUM payload size it can support in both direction since it can not guess what are the capabilites of the receiver. The receiver has to "select" from the list provided in the SETUP.

Suppose that A calls B using fast start. A has a big buffer and can support G.711 at up to 100 ms (send and receive). B can only support 20 ms. A calls B using fastStart and advertizes 100 ms for both direction. B has no choice but to reject the call, or to initiate H.245 tunnelling or a separate H.245 channel. This means to me that it can actually be a disadvantage to advertize a bigger maximum payload size than the default in fasStart. It also mean that fast start only has a reasonable chance of working well if by chance the called party can handle an equal or larger payload size than the originator.

One possibility is that the receiving endpoint would be allowed to signal a SMALLER payload size in the OLC it sends back to the originator. This would however mean a change to the specification, and would potentially mean backward interoperability problems since current implementations may not accept OLC contents that do not match what was originally sent in the OLC in the SETUP message (they might reject the call, or might ignore the changed values).

The other possibility is for the sender of the SETUP message to advertize multiple payload size as alternative OLCs. While it would be possible in some cases to completely describe one's capabilities without creating huge fastStart messages (eg., G.723-30 ms, G.723-60 ms), it may not be practical for others (e.g., G.711-100 ms, G.711-99 ms, G.711-98 ms, ... down to 1 ms). This would mean that in some cases, you might be restricted to only a few (maybe advertize the maximum and the default). Maybe a trick would be to advertize always at least the default (20 ms) if supported, and the maximum if different.