Expansion? They're down one Silesia and up net one Banat of Temesvar and Galicia and New Galicia for the century, as compares with say, Alta California, claims further northwest, and Luisiana for Spain. While the latter are more vast and have greater potential, the former in Europe have more taxable subjects in the here and now.
So I guess your argument reasonably checks out, but it's close. I guess the biggest balance tipper would be the WotSS transfer of Spanish Netherlands (and Milan?) to Austria.
Expansion, or contraction depends on when you want to check in to see how things are going. A hundred years is a long time. At different points, either country could be going up, or down. Same for diplomacy.
WoSS saw a massive loss of territory for Spain. Same with WoAS for Austria.
Technically, Spain didn't gain anything back.
I would put Austria well above Spain in the diplomacy department.
In WoSS Spain wasn't even consulted in the partition. Austria, while being forced to accept limited gains, and forced to cough up Bavaria, did have a say in the outcome.
Spanish diplomacy caused a breech with France during WoQA and France's regency period in general.
Anglo-Spanish war saw some laughably botched diplomacy in making an alliance with Austria. Spanish diplomacy led to a breech in relations with France just prior, depriving them of a potential source of support.
WoPS saw poor diplomacy with Savoy, resulting in far less gains than could have been had.
Spain was barely consulted at the peace table in WoAS.
Britain had some willingness to talk about returning Gibraltar, but Spain could never accomplish it.
Spain botched the diplomacy leading up to WoJE, leading to an avoidable war. Spain negotiated a defensive alliance with France that turned out to be useless in this war.
Spain was eagerly used by France in several wars, such as WoPS and WoAS.
Ferdinand wisely stayed out of 7YW, but then Carlos foolishly allowed himself to be maneuvered into joining the losing side AFTER the war was basically over.
Spain couldn't get any help in both the Falklands Crisis and Nootka Crisis.
Spain's diplomacy following the French Revolution caused France to declare the War of the Pyrenees.
Spain was used, abused, and pushed around by France after War of the Pyrenees, although, to be fair, their bad loss in that war left them very little room to maneuver.
ARW was Spain's shining moment of the century. They extracted concessions from France, and wisely limited their role, while France was being played like a fiddle by the Patriots.
Austria, despite the foolishness of garnering support for the Pragmatic Sanction, was mostly competent throughout the century. They played France like a fiddle in the Diplomatic Revolution. They did botch the diplomacy during the Partition Treaty talks, leading them to be excluded. They managed, with one brief exception, to provide the Emperor for the Holy Roman Empire. And while they didn't always get their way, they were always sitting at the peace table during their wars, something Spain could only accomplish on occasion.