You need to have
'excerpt' => 'yes',
'excerpt_size' => '100',
'content' => 'no'
… when get_excerpt runs. You can set $params like that but what you posted is not all of the code. It is using $this which means it is part of a class (or you’d get errors). Somewhere in that class something might be altering the defaults, and I suspect something does. If that doesn’t work you need to edit your question to provide more of the code.