I like to think I’m not sizeist. As far as I’m concerned, what people weigh is up to them. But when it comes to opera? I’m beginning to think I might be.

At Raymond Gubbay’s Aida in the round at the Royal Albert Hall last weekend – a fabulously staged theatrical experience complete with pyramids, tombs, sphinxes and archeological remains – several of the soloists and chorus members looked to me as if they were distinctly out of shape. Their singing, however, was faultless (as was the playing of the RPO). So did it matter?

Well, in a word, yes. When a singer fails to look the part, our ability to transcend the confines of the concert hall or opera house and enter the realms of our own, and the composer’s and director’s, imagination can become hindered. In this case, my two main niggles were straining costumes and, at times, clumsy, verging-on-comical movement around the stage. The result was that my focus started drifting away from the narrative and the music as my belief in the characters began to flounder.

There’s an important point to make here. Larger singers can, and frequently do, convince in their acting abilities and therefore in their roles. The director takes into account how they look and move, and works with them to draw the best out of them. If a singer isn’t automatically a match for a role visually and physically, their movement, demeanour, make-up and costume can still create an illusion. (This works the other way, too, with slim singers who are playing physically larger characters – and again, if they are not given the right direction, the effect can be far from realistic.) And even if a singer does seem physically perfect for a role, any shortcomings in the acting department can still be disastrous. But, in the case of Aida, I felt that singers were not given the right direction to overcome any physical limitations imposed by their size. Similarly, I didn’t feel that the wardrobe department created outfits that flattered as much as they could have. Acting ability has always varied among singers but, again, with the right direction and coaching, any shortcomings should be able to be overcome. Here I didn't think they were.

It used to be the case that opera singers could look how they liked – it was their voices that counted. But the tide has been turning in recent years with the physicality of such opera singers as Natalie Dessay, for whom belting out top Cs while ironing and peeling potatoes is effortless (it helps that she’s a consummate actress – in fact, acting was her initial career choice). Her role in Laurent Pelly’s La fille du régiment at the Royal Opera House changed the way I, and many others, viewed opera singers. As she once told me in an interview, it’s no longer acceptable to just ‘park and bark’.

Other singers seem to be embracing this notion, too. I recall a lithe Kate Royal practising yoga in a bikini as she sang her opening aria in ENO’s The Coronation of Poppea. And my breath was fairly taken away during Nicholas Hytner’s perfectly cast Così fan tutte at Glyndebourne, in which Miah Persson, Anke Vondung, Topi Lehtipuu and Luca Pisaroni looked, and sounded, exquisite. Their acting and movement were so convincing that I almost wouldn’t have minded if there hadn’t been any singing at all. As The Independent wrote at the time, ‘The simmering sexual attraction on which the plot depends is entirely believable.’

And I suppose that’s my point. We all know that, on the whole, opera plots can be far-fetched, and at times, frankly ridiculous, but if we are to throw ourselves wholeheartedly into the experience, the singers involved need to convince us that they believe they are who they say they are. Sarah Connolly in ENO’s recent Der Rosenkavalier, for example, had great fun in her role as Octavian – and at the same time never left us in any doubt that she was a young, besotted male adolescent (albeit with a penchant for cross-dressing).

Back in 2003, Deborah Voigt hit the headlines when she was sacked by Covent Garden for being too fat. That summer, she had gastric bypass surgery; today, she’s some 15 stone lighter and has been welcomed back to the Royal Opera House. Is she bitter about the experience? From interviews she’s given, it seems not. Her health and mobility have improved and, as she herself has said, ‘I'm not sure I would have wanted to see me, 150lbs heavier, singing Salome. I think opera houses have to compete for entertainment dollars just like anyone else…I would like to believe that the most important thing in opera is the voice. But at the same time it's a business.’

Some opera lovers have argued that Voigt’s decision to slim down was wrong. With the quality and timbre of the voice being so dependent on the physical support of the stomach and diaphragm, there is a danger that a slimmer body equals a slimmer voice. And in recent years, some critics have spoken of Voigt’s voice becoming more hard-edged and less warm. Voigt herself is non-committal: ‘I don’t think the size of my voice has changed,’ she has said. ‘Maybe it’s a little brighter, more silvery than gold.’

Voigt has spoken out against the sexism inherent in all this – that women on stage are criticised more regularly for being overweight than men. But for this writer, it has nothing to do with being male or female. Yes, in opera, the voice is paramount and nothing must get in the way of that – certainly not surgery, unless it’s for health reasons as it was with Voigt. But the ability to convince the audience by embodying a role comes a close second. And if I’m not convinced, then I’d rather not bother. Go on, shoot me.