  We study the phenomenon of length scale competition, an instability of
solitons and other coherent structures that takes place when their size is of
the same order of some characteristic scale of the system in which they
propagate. Working on the framework of nonlinear Klein-Gordon models as a
paradigmatic example, we show that this instability can be understood by means
of a collective coordinate approach in terms of soliton position and width. As
a consequence, we provide a quantitative, natural explanation of the phenomenon
in much simpler terms than any previous treatment of the problem. Our technique
allows to study the existence of length scale competition in most soliton
bearing nonlinear models and can be extended to coherent structures with more
degrees of freedom, such as breathers.
