Alan,
All brass is not created equal! You put your finger on the difference, the Starline Brass is thicker! It is even thicker than Remington brass.
Try weighing a Starline case, then compare the weight to that of a Federal, Winchester and Remington of the same cartridge designation. You'll find the Starline brass much heavier! It surely isn't your imagination.
Now, about what to do. Just keep in mind, if there's more brass, and it's thicker, that it won't have the same internal case capacity of brass of thinner construction. Consequently, you'll have to watch your loads, and work up as pressures indicate. (A top-end all out max load in a Federal or Winchester case will most likely be well into the red-zone of hazardous when loaded directly into a Starline case)
Now, for an up-side to all this! Starline is some of the most uniform of new brass being manufacured today. It is extremely uniform not only in dimension, but weight as well when compared case to case, and even lot to lot of brass. Too, the brass they use is of very high quality, and the life of Starline Brass is excellent! It can be loaded many, many times without failure, cracked or split case mouths or incipient head separations (provided proper headspacing and die adjustment) Starline is great brass, make no mistake about it. For most everyday loads, Starline is a logical choice, the brass is reasonably priced, and it is always available.
FInally, because of the somewhat limited case capacity of Starline brass, it isn't necessarily a good choice if you are looking for top end loads, due to it's somewhat impaired case capacity. Generally if you are doing top-end load development, either Winchester or Federal is the brass of choice, due to its typically thinner brass and greater case capacity for a given cartridge.
The last word... Starline is great stuff, don't worry about the thickness of the brass, especially for mid-pressure loads. Also, consider it a blessing having the thick brass if you shoot a Ruger revolver... it will help fill up the genereous chambers that Ruger cuts in their cylinders. :smile:
Hope this helps!
God Bless,
Marshall