sometimes the sizes are correct when xsi creates unique UVs, then all parts fitt ecactyl together, but lately the proportings and sizereleations of the parts arent correct.
how can i make xsi doing UVs with correct proportions everytime?
or what did i do wrong when xsi didnt make them proportional?

Sub-projections usually work great. It's what I've been using lately to quickly UV things and make them look nice.

i know, i use subprojection all the time, but sometime i had to subproject nothing because everything fit with the standart unique UVs, now i want to know "how and when" xsi creates them
fi you use subprojection you still have to look at the correct sizerelations between the UVparts

You can use the default noicon.pic or a checkerboard style image for proportion reference and manually scale the UV islands in the TE while watching the size of the squares(checkerboard) on your model in the 3d viewports.

If you have the a Full version of XSI, try using Roadkill too, its way helpful in starting layouts for complicated or simple models, and only small adjustments are needed if you made your cuts well.

Its already been talked about here and is in the XSI Tutorials sticky.

http://www.pullin-shapes.co.uk/page8.htm