Page "Multiple frequency-shift keying" Paragraph 18

Because a long symbol contains more energy than a short one for a given transmitter power, the detector can more easily attain a sufficiently high signal-to-noise ratio ( SNR ).

The resultant throughput reduction can be partly compensated with a large tone set so that each symbol represents several data bits ; a long symbol interval allows these tones to be packed more closely in frequency while maintaining orthogonality.

This is limited by the exponential growth of tone set size with the number of data bits / symbol.