The size is intentional; the positioning needs fixing, though.

We need to reduce the size of this callout. It wasn't really designed in context, so now that we have context we can finally properly adjust it. I think it's about 2x too big.

@Pginer-WMF Do you think the anchor/callout/triangle is oversized in MediaWiki theme with its base side width of 10px. I've always thought it's a tad too prominent and should probably be reduced to 8px.
In the OOjs UI documentation Apex theme is still featured as default theme, it features a 7px base side width.
I propose to go for a 8px base side width.

Current (as of v0.19.4):

8px:

WFM.

In T85949#3076396, @Volker_E wrote:

@Pginer-WMF Do you think the anchor/callout/triangle is oversized in MediaWiki theme with its base side width of 10px. I've always thought it's a tad too prominent and should probably be reduced to 8px.

The proposals look good to me, but I wanted to point out that there are different measures involved here that we may want to specify and make conscious decisions about: length of the sides, angles, and separation from the object. For example, if you aim at a 8px vertical rithm (maybe we don't need it here), the triangle in your example despite using an 8px side as a base is going to break the rithm because of Pythagoras.