The HRE had gone for nearly a thousand years without tons of small minors uniting into larger nations, but as the French Revolution began, Austria, Hanover, Westphalia and Prussia all either merged into existence or expanded greatly across the empire. What caused the dramatic change? Was it merely the threat of the French revolutionaries that made minor states willing to join into larger ones?
First off, all of these states existed in one form or another before the French Revolution, Westphalia excepted. Hannover used to be the electorate of Braunschweig-Lüneburg, but was already inofficially known as Hannover. Westphalia as a territorial entity was mostly formed from Hannoverian lands, with the addition of Hessen-Kassel and parts of former Münster and Braunschweig-Wolfenbüttel plus a few others and was dismantled after the Congress of Vienna.
Now, the cause for all this. What happened? The French annexation of the western Rhineland happened.
One of the secret clauses of the peace treaties with Holy Roman Emperor Franz II. and Prussian king Friedrich Wilhelm III. was that their losses west of the Rhine were to be compensated in the east. The final plan for this (the 1803 Reichsdeputationshauptschluss), which was worked out by the major HRE princes in conjunction with the Emperor, stipulated that all territories held by spiritual entities as well as most of the free cities (the only survivors of about 51 were Hamburg, Lübeck, Bremen, Augsburg, Nürnberg and Frankfurt a.M., however Nürnberg and Augsburg were annexed by Bavaria in 1805/1806) were to be annexed by the larger HRE princes around them, for example the bishoprics of Münster, Hildesheim and Paderborn being taken over by Prussia in exchange for losing the area around Kleve.
The funny part is that of those you mentioned, only Prussia gained major amounts of territory. Austria had lost the Low Countries and the Vorlande as well as the important support of the now-gone smaller estates in the Empire at the Reichstag (and only gained two minor bishoprics in exchange), Hannover only gained Osnabrück (and got temporarily annexed by Prussia a few years later) and Westphalia only existed as a polity in the early 19th century. The main beneficiaries in addition to Prussia were the pro-French southern German states, mostly Baden, Württemberg and Bavaria, which gained triple to octuple(?) the amount of territory and population they might have lost west of the Rhine.
Then, when Napoleon fell (going to skip over the wars itself because the outcome is the important bit), came the Congress of Vienna, which restored much of the western losses, however without returning the territory that was given as compensation and in fact even integrating some of the lands that had been minor HRE princes or belonging to other HRE nations before the French annexed them, such as Prussia receiving what had been Cologne and Jülich-Berg (which had been Bavarian) before, in addition to regaining much of Kleve and the compensatory lands they had received in 1803, now controlling the largest part of the Rhineland. Hannover regained its territories which it had lost after being dismantled in 1806, plus northern former Münster, Hildesheim and Arensberg from Prussia and was made a kingdom, as were Bavaria and Württemberg earlier. Austria's gains were mostly to her south, in Italy, and to the east, while the only German territory it received in Vienna was Salzburg.
Do note, however, that at that point the HRE was already gone.