Can a flue be too big?

Hi all me again and I’m still trying to get my rmh right it’s only a small metal one at the moment but once I get it right I will build a more permanent one
So my question my air inlet is 100x100 mm2 and the rmh does get hot around 400c at the top my flue is currently a uninsulated 125mm flue liner and although it gets hot ish when I make it smoke it just seems to flow out as apposed to quickly coming out do you think the flue pipe is to large thanks all

From the dimensions that you give the size probably isn't the issue. I'd guess that there is some combustion issue and may be as simple as your flue is uninsulated, or isn't long enough, or doesn't go high enough over the roof line, to provide good draft.

Hi Mike,   When you say "flue pipe" do you mean the heat riser or the chimney outside the building? Both need to be insulated. Could you include some pictures?