I'm working to find my exercise and calorie balance for maintenance and thought I'd found it. I was eating 1650-1700 most days, exercise 5 days per week with three days of 45 min steady cardio, three days of weight training plus a shorter, slower cardio or some consistent intense cardio. I maintained 121.7 for my 10/10 weigh-in, went on vacation for a week and blew all my calories out of the water (2000+ three days, 1700-1800 the other days) but still exercised, was at 121.5 on 10/22 once I got rid of the water retention from the bad food and had a couple of on plan days. Today I weighed 120.2, so I've lost more weight in the last week. I'm still eating 1700 calories, sometimes a little more, the only difference is HIIT versus lower intensity interval cardio, which I actually started while I was on vacation on 10/14. I had been doing intervals on the long cardio days, but never to the high heart rate of HIIT. I've shortened those days to 30 minutes and am actually doing the HIIT correctly, now. My pants are significantly more loose than two weeks ago, too. Can doing HIIT instead of the steady or interval cardio really have this much of an impact?

I think it absolutely can, considering the elevation in metabolic rate that comes with HIIT compared to lower-intensity forms of cardio. Not to mention the higher number of calories burned during the workout.

Another thing to take into consideration would probaby be the fact that you significantly changed your workout routine, which I personally find takes weight off of me quicker than sticking with the same thing, and my body isn't used to the workout.