I actually would say they were tiny, I haven't used the Jays or UE700, the smallest IEMs I own are the Nocs NS800, 200 mm long from end of shell to end of tip and an 8mm diameter barrel. The RE400 are 202 mm from shell end to tip end and 100 mm diameter at it's widest point (7 mm at its narrowest).
I do want to update with sound impressions, they've gone through about 24 hours of burn in so far with about 4 hours of total use. I have to say I'm impressed with them so far, they are definitely pushing on the analytical side of things. Low-to-mid-bass focus, great texturing, but lacking presence down low. Midrange is highly detailed with great vocal dynamics (something I remember the RE0 lacking) while the highs are extremely detailed (analytical to a point) with high extension. It's an IEM, that like many of HiFiMan's offerings, tries their best to reach neutral response, it does a decent job of it as well. The IEMs do lack a warmth to them though :( Highs are more strident than the RE0 (which I felt was an extremely perfect balance with hybrid tips).
*Note, I'm still using the single-flange tip that came with it. The dual flanges boost the lows, but also constrict the highs for me. I'm still playing with tips though, so things may change.
OK, I'll compromise and say they are "really small" but not quite the smallest I've seen. Semantics really, but they should make for an easy and comfy fit for most ears.
I'm hearing them somewhat similar to you except for the bass response which to my ears hits decently hard and extends satisfyingly low. I'm using the dual flange or else a set of frankenstein tips - large UE single flange covering a modified comply, which gives me a super tight seal without any comply drawbacks. In both cases - but especially the tighter comply fit - I'm very satisfied with extension on both ends.
Like you, I do find them leaning towards slightly analytical especially in the vocal range, but overall they seem pretty balanced to my ears rather than thin which is what I recall from past RE models.
I'll try out the single flange tip and see how that works.