Why?

True, I can understand that being the case with most other late-romantic composers. Sibelius, however, should be something of an exception. Compared to Mahler, Sibelius' orchestra is tiny and his symphonies rarely exceed a forty-five minutes in length (most average about thirty minutes total, and his tone poems half even of that).

Ah well. Either way I always classified Sibelius as a "modern" composer rather than late-romantic, though many will argue about this. In addition to practically creating Finland's musical voice, he helped expand the concept of the symphony and the organic development of (often highly condensed) themes. I'd say he's had more of an influence on my composing than more contemporary composers such as Stravinsky, Shostakovich, or Glass/Adams. Of course, it could just be the Finnish blood in me.

Interesting... I will give Sibelius credit for what he did with the symphonic form, but I always considered Brahms to be the first great master of organic development of themes.