Currently, whenever Simutrans Exp 11.35 is started, the number of rivers in the Climate Settings dialog is set to 128 always. It should instead depend on the map size.
This is because 11.35 is based on an older version of Standard; the current development version (the way-improvements branch on Github) incorporates this feature from Standard and the number of rivers do indeed (by default) proportionately increase with the size of the map.
Oh ok, thanks!