The debate over size flared last year when researcher Chant West said over half of the default funds were too small and should merge as they do not have the economies of scale to keep the fees low.

Chant West defined "sub-scale" funds as those with less than $5 billion in assets.

One would expect there to be a strong positive correlation between size and performance.

However, SuperRatings' figures published on the SuperGuide website shows a large variation in the performances of the superannuation giants' balanced investment options, where most fund members have their money.

And it is not just big variations over the short-term.

Industry fund REST's Core Strategy produced an average annual return of 6.7 per cent over the 10 years to January 31, this year. That makes it the best-performing balanced investment option of all.

The Core Strategy is also among the best-performers over seven years and five years; though its performance drops away a little over three years.

Other strong performers over most time periods include AustralianSuper Balanced, UniSuper Accumulation (1) Balanced, CBUS Growth and Telstra Super Corp Plus Balanced.

Poorer performers over 10 years include retail funds such as Mercer Super Trust – Mercer Growth with 4.7 per cent, OnePath Corp – OptiMix Balanced (owned by ANZ) with 3.5 per cent and AMP FLS – AMP Balanced Growth with 3.9 per cent.

They also tend to have poorer performances over shorter periods.

The big not-for-profit funds include industry funds, public sector funds or in-house corporate funds such as that for Telstra employees, ex-employees and their families.

These are the balanced options from among the 20-biggest funds, those with total assets of at least $17 billion, and often much more. They should be benefiting from economies of scale.

They are probably are. Not only are their costs per member lower, but they can invest in a wider set of investments that small funds cannot.

So what is going on? The most likely explanation is that retail funds are holding onto more of those benefits of scale for their shareholders.

The poorer performing large retail funds tend to have higher fees than do their not-for-profit rivals. That is changing, however, with retail funds' fees getting lower.

Another likely factor is asset allocation.

Industry funds, for example, tend to have a higher exposure to unlisted assets such as infrastructure.

Industry funds have also been more active in asset allocation and prepared to adjust their asset allocation as markets change than retail funds.

While fund members should be focussed on their funds' the longer-term performances, shorter-term performances also matter.

That is because the returns of a good performer can drop off just as a poor-performer can improve.

Returns are very important in choosing a fund. Insurance offerings, administration and customer service are also important considerations.

Twitter: @jcollett_money