The relation between the fundamental parameters of energy and temporal duration of ultrashort pulses, under the condition of varying the average dispersion, are demonstrated both theoretically and experimentally in a solid-state femtosecond mode-locked laser. An asymptotic theory for nonlinear and dispersion managed solitons agrees well with the experimental data and demonstrates that the dominant factor in the pulse dynamics arises from the equilibrium established between the nonlinear Kerr effect and linear dispersion.