Asked to be an external reviewer of a grant application, I think the proposal looks good, but I don't know the small field well, though I'm in the same big field. So it's a bit hard to write comments. How should I write comments?
Consider asking for guidance from the grant manager(s) on this. They should have some guidance regarding the style, content, and formatting of comments. Further, as you sound as though you are a little unsure about your ability to comment on and, thus, evaluate this grant. This is something else to share with the grant manager(s) as they may decide you are not the right person to do this. Thus, best thing to do is contact whoever asked you to review the grant and have a chat about this.
Perhaps, you were engaged as a reviewer precisely because you are an outsider (to some extent), or what the Leverhulme Trust would term a "lay reader" (well, maybe not quite, but the idea is similar). If so, I would say your function includes:
challenging unstated assumptions shared by people inside the field but which may not have been subject to proper scrutiny (it is incredible how much people take for granted);
assessing the potential for wider significance from the research;
identifying relevant parallels with research in other fields, especially regarding methodology or critical frameworks (often, a researcher may be unaware that a similar methodological problem has been discussed thoroughly in the literature in another field, and would benefit from taking such discussion into account).
A lot of the time, you may be unsure whether a problem is due to your misunderstanding, a flaw in the proposal, or a general bias of the field. That is fine. Your colleagues on the review panel expect you to raise issues and seek clarification in the course of your panel meetings/discussions. Sometimes, there is a simple explanation and you move on; at other times, you and your colleagues may wish to probe the issue further, and raise it with the people who wrote the proposal.
[personal experience: I was reviewer for a degree-programme revalidation, in a subject that was not my expertise. There were two of us subject "outsiders", and we questioned some practices that we found counterintuitive. I raised an issue that was then taken-up by the whole panel and discussed thoroughly. The faculty who ran the programme being assessed gave some excellent explanations in response, and convinced us of the merit of their position. We asked them to articulate this reasoning more explicitly in the documentation, but otherwise approved the programme.]