Candidate Scares Recruiters

Question from Candidate:
I have recently graduated and moved to the UK from the Netherlands. I’m actively looking for a job. I have no trouble getting invited for interviews, and I am often invited for second interviews as well. Not so long ago I have been rejected for one of the jobs I applied for on the account of that the interviewers found me “to confident of herself”. This is a reoccurring thing. My confidence and assertivity overwhelms people. I’ve heard this since nursery school. It’s not that I talk very loudly, or throw demands in people faces, I don’t jumps queues, I do not look down on people or offend people in any other way. I am just reasonably aware of things I can handle, things I need to learn, how long it will take me to learn them, what needs to be done when people start panicking etc. This means I’m mostly fairly at ease in every situation.
After hearing this from the recruitment agent that introduced me to the company I decided to tone down a bit for my next interview with a different company. I brought up my dominant personality as one of my weak points and explained that I’m aware of it and try to manage it. I thought the interview went really well, and upon leaving the HR manager confirmed this and let me know they wanted to invite me back for the second interview. After this I got a phone call from the recruitment agent, who had some feedback on the interview. She told me the HR manager, in the beginning of the interview, was actually scared of me…
I was kind of in shock to hear this. I deliberately wore a pastel cardigan, spoke in a soft voice, said thank you and please a lot, etc. I asked questions in an interested manner and tried to not sound bragging when my skills and expertise came up, but gave examples. Apparently after 15 minutes she warmed to me, and wants to see me back for second interview.
I am Dutch and therefore naturally taller then most English women and a bit heavyset as well, which will naturally contribute to me being scary. However, it apparently is mostly my personality and my way of speaking that is most frightening, as I have heard this from Dutch people as well. I hope you can give me some advice on how to overcome this problem. It didn’t bother me much during university, but now it is costing me my career.