Why would a planet form a cometlike tail under conditions of weaker rather than stronger solar wind?
I think the first link that you sent has an interesting explanation that we can extrapolate from - here's the relevant section:

The new observations settle a debate about how the strength of the
solar wind affects the way in which ionospheric plasma is transported
from the dayside to the nightside of Venus.
Usually, this material flows along a thin channel in the ionosphere,
but scientists were unsure what happens under low solar wind
conditions. Does the flow of plasma particles increase as the channel
widens due to the reduced confining pressure, or does it decrease
because less force is available to push plasma through the channel?
“We now finally know that the first effect outweighs the second, and
that the ionosphere expands significantly during low solar wind
density conditions,” says Markus Fraenz, also of the Max Planck
Institute and co-author on the paper.

What I interpret is this: while solar winds do cause plasma to form a tail, or stream, a very strong solar wind will limit how voluminous that tail can be. Thus, a slightly weaker solar wind would have less of a limiting effect on the volume of the flow of plasma.
This pressure-reducing effect outweighs the effect of there being less force to actually move the plasma - as said:

“We now finally know that the first effect outweighs the second, and that the ionosphere expands significantly during low solar wind density conditions&quot;

So, this comes down to which effect is stronger - is it the expansion of the tail due to less confining pressure, or the decreased force pushing the plasma into a channel/tail? Observationally, it seems to be the first.
Though, from here, I would also assume that there is some sort of lower limit - or equilibrium - at which the size of the tail/channel reaches its maximum. If the solar wind were far too weak, there would likely not be a tail at all. So, rather than &quot;lower solar wind causes a larger tail&quot;, I think this phenomenon would be better phrased as &quot;excess solar wind can decrease the size of a comet-like tail, specifically around planets&quot;. It's not that lower solar wind causes a larger tail, it's that there is a sort of mid-point at which the solar wind creates optimal pressure and conditions for the tail to reach its maximum size.