The reason for this is the setting in the bootstrap css
.EditTable td input,
.EditTable td select,
.EditTable td textarea {
width: 98%;
display: inline-block;
}
i.e this is caused from width:98%
To correct this there are a lot of ways.
The first one is to use editoptions to set a manual auto width with style options.
edittype : 'checkbox',
editoptions : {value:"true:false", "style":"width:auto"},
The second one (more elegant IMHO) is to set this in css for all check-boxes in the edit form like this:
<style>
.EditTable td input[type="checkbox"]
{
width :auto;
}
</style>
after loading the jqgrid bootstrap css file.