Why is that some high alcohol by volume (ABV) wines can taste lighter than expected ?

When buying a wine from an online store or shop we may expect (in reference to red wines) bottles with an ABV of 14% to be heavy, textural and slightly viscous on the palate, but this is not aways the case and there are a number of factors that  add to "Mouthfeel" beyond just alcohol content:

Tannin and body - Wines low in tannins and with less viscous texture can come across as lighter, even if their ABV is high. Body (viscosity and mouthfeel) is influenced by alcohol but also by glycerol and residual sugar levels.

Residual Sugar - Some wines retain some residual sugar, which can create a perception of sweetness that balances alcohol heat, making wines appear softer and lighter. A good example of this would be Mucho Mas Black edition from Spain

Winemaking Techniques - techniques like cold fermentation and limiting maceration can preserve fruit brightness and acidity, which counteracts alcohol’s weightiness. 

Grape Variety and Climate- Some grape varieties naturally produce lighter wines with higher alcohol, especially when grown in cooler regions where acidity stays high despite ripeness. A higher altitude Malbec from somewhere like Salta,Argentina is a good example of this. I.e El Porvenir Amauta Malbec

Perception - The way alcohol is perceived varies from person to person and depends on many factors including time of day, prior food consumption,altitiude,types of wine,mood, flavor profile, and serving temperature. if you try a New Zealand Sauv blanc at High altitude (2,000 masl) it will most likely taste different to that at sea level

Even if a wine has high alcohol content, if it maintains high acidity, light tannins, balanced sweetness, and good freshness, it can taste lighter and more refreshing than its ABV might suggest. Alcohol contributes to body and warmth, but these other factors significantly influence the overall mouthfeel and weight perception.

This explains why some warm-climate wines with high ABV can feel full and heavy, while others, especially aromatic whites and balanced reds, can feel surprisingly light