Nice catch, hope they can fix this one easy.

Sometimes the proof is in the hull size. Perhaps on bigger hull size the Droid III takes less space than II ? Yes? I've seen it with other techs but havent checked these two.

No, some components get bigger with the hull-size. Probably doesn't do a difference here though since you use a huge hull but it's easy enough to check.
Click on Defenses without choosing a hull and you will get the size unmodified by hull-size.
If they still are identical, then it's a bug.

{Name}DroidSentries2{/Name}
{Class}PD{/Class}
{Size}4{/Size}
{SizeMod}2{/SizeMod}
{Cost}100{/Cost}
{Absorption}6{/Absorption}
{DisplayName}Droid Sentries II{/DisplayName}
{Description}Missile defense.{/Description}
{Model}pointDefense0{/Model}
{Thumbnail}pointdefense{/Thumbnail}
{Category}Point Defense{/Category}
{Tech_Requirement}Droid Sentries II{/Tech_Requirement}
{Animation}Defense.x{/Animation}


{Name}DroidSentries3{/Name}
{Class}PD{/Class}
{Size}4{/Size}
{SizeMod}2{/SizeMod}
{Cost}100{/Cost}
{Absorption}6{/Absorption}
{DisplayName}Droid Sentries III{/DisplayName}
{Description}Missile defense.{/Description}
{Model}pointDefense0{/Model}
{Thumbnail}pointdefense{/Thumbnail}
{Category}Point Defense{/Category}
{Tech_Requirement}Droid Sentries III{/Tech_Requirement}
{Animation}Defense.x{/Animation}


As you can see here, they're absolutely identical.

(Nice catchu! )