When placing one portal immediately in front of the other, the portal graphics of one move or vanish. Most noticeable with a large portal in front of a small portal, it looks like the portals render closer together, rather than in the middle of the block.

Describe what you thought should happen here: I expected the portal surfaces to look smooth on each side

What actually happened was... there was an "indent" on the larger portal, where it met the smaller portal

Step by Step:
create a regular size portal
create an oversized portal one block behind.
look through the larger portal to where the smaller portal is,