The news that cars have been getting steadily larger over the last decade or more will not come as a surprise. With increases in weight and dimensions, engines might have been expected to get bigger, too.

This has not been the case. More advanced engines and technology have allowed car-builders to get more power from familiar capacities without sacrificing reliability. Rather than increasing in size in parallel with vehicles they power, they have merely increased output.

Now the things are going even further. While cars continue to grow in stature, the engines powering them are shrinking in size.

Take Audi's A3 Cabriolet. Leaving aside the folding roof for one moment, the A3 is a vehicle that competes alongside the Volkswagen Golf, Vauxhall Astra and Ford Focus – albeit aimed at a more premium market.

Take examples of these cars from 15 years ago (in the case of the Focus you'll need to think Escort) and the A3, like its contemporaries, dwarfs them.

Despite this, Audi's A3 Cabriolet can now be equipped with what seems like an outrageously small 1.2-litre engine. That's even more remarkable where the cabriolet version is concerned as, despite the canvas roof, the model suffers from the same weight issues as nearly all convertibles thanks to the extra motors and chassis-stiffening needed, and weighs nearly 100kg more than its three-door equivalent.

Audi, along with its Volkswagen Group stablemates has 'previous' when it comes to setting trends with small-capacity engines. Previously, it was the 1.4-litre unit, available in turbocharged and twin-charged (supercharged and turbocharged) form, which raised eyebrows with its capacity belying performance.

The 1.2 TFSI unit also uses forced induction to boost performance to respectable levels while keeping fuel consumption and emissions low. A replacement for the 1.6-litre unit, it lowers CO2 emissions by 35g/km and raises fuel economy by 9.2mpg while improving 0-62mph acceleration and top speed.

It's a highly-charismatic unit that adds a lot to the A3 Cabriolet. Both compact and sophisticated in their own rights, car and engine seem made for each other. Much of the charisma comes in the form of the engine note; a distinctive grumble that's ever-present but rarely intrusive. With the roof down it adds to the driving appeal, offering a sports-inspired – if not sporty – soundtrack.

If it's outright performance that's sought, perhaps the 1.2 TFSI is not the best choice, but considering the impressive real-world economy and tax-lowering emissions it does a good job of shifting the A3 Cabriolet. Despite the turbocharger, power does not arrive in lumps of high boost, instead being evenly spread across the rev-range.

The chassis is impressively rigid, too, with scuttle-shake contained roof up or down and little flimsiness apparent at the wheel.

To really gather pace, the 1.2 TFSI A3 Cabriolet needs to be worked fairly hard, but a standard six-speed manual transmission means there's enough choice to ensure the correct ratio can always be found. A longer sixth helps with motorway cruising. The little engine is comfortable at motorway speeds with impressive refinement, although a little wind noise can reach the interior.

The A3 Cabriolet really shines away from the motorway. The smallest-capacity engine is at home in town with impressive urban fuel consumption and slick, easy-to-drive nature. With the roof down, it's chic, smart and sophisticated.

Where Audi has succeeded with its smaller cars, including the A3, is the grown-up feeling inside, reminiscent of the larger executive models.

In Sport trim the A3 Cabriolet has an executive appeal. Varying colour options for the quick-fold, one-touch electric roof add to the upmarket feel as do impressive materials and impeccable fit and finish inside.

Compromises come in the form of a small boot opening, restricted rear seating and a slightly firmer ride on the larger alloy wheels, but as a complete package the model offers a level of style and quality that's rare in the sector.

Add increased economy and the A3 Cabriolet's appeal gets bigger and bigger while the engines get smaller and smaller.

Audi A3 Cabriolet 1.2 TFSI Sport

Price: �22,800

Engine: 1.2-litre, 103bhp four-cylinder petrol

Transmission: Six-speed manual, driving the front wheels

Performance: 0-62mph 12.2 seconds; top speed 118mph

CO2 emissions: 132g/km

Economy: 49.6mpg