I have a couple of Pyreneans, and at 58kg (female) and 68kg (male) they're not small dogs! Over the last few months I've found medium-large dogs (German Shepherds, Boxers, now a Lurcher) are very keen to get a piece of them, and we've changed our walking times repeatedly to avoid them. My two have very good temperaments and usually ignore most dogs and people, simply walking past them. Even when small dogs repeatedly bark at them they usually just back off. Unfortunately, it's not so easy for them to walk away from larger dogs which, I suppose, they realise could pose some threat. So, they usually back off a bit then stand their ground, waiting for it to make the first move. They usually do, so either a chase or rarely, a scrap starts.
I usually feel the bad guy, because although there have been no fights to cause damage, as the other dog usually turns tail pretty quickly, mine invariably gives chase and could do far more damage if she tried. This seems to be a fairly common perception - that the bigger dog is the trouble-maker: only a few weeks ago, we passed a Collie (my two and it on leads) and the owner pulled the Collie over to the side when it raised its hackles and started growling softly (we've met it before - it always does this, although mine rarely notice it!). Another dog walker who saw mine earlier and was nervous, then commented to the Collie owner that she seemed to have pulled it over to avoid the Collie meeting dogs it would be afraid of, and was surprised when the Collie owner admitted it always wants a slice of THEM!
Do other people with giants find this sort of problem, that reasonable-sized dogs are more aggressive towards them than other dogs? This doesn't seem to apply to labradors or Golden Retrievers though!

Malamutes aren't Giants but they are big dogs and we find the little ones want to have a go at Hudson :)

Maybe they're jealous because Hudson is such a stunner :D

I've noticed this with my parents Cocker - she gets really stroppy with big dogs, almost as if she's got something to prove but she's great with the littler dogs - maybe because she feels she can dominate them. Holly on the other hand loves all dogs now, and will play with anything in the park.

Fiona
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Goodmorning
I have bullys and I have always found it difficult walking my dogs in places that other people walk theirs as other dogs want to fight the bullys. I think its something to do with the fear thing. Scared dogs want to fight.
Mick

Hi
I have a 2 year old male (entire) Leonberger and have exactly the same problem. I usually find that the smaller the dog the more aggressive it's behaviour towards Rory!
The worst thing about it is that Rory is so gentle (and stupid) that he doesn't read the dog language telling him to go away and he always wants to make friends and play with other dogs. Like you I know he wouldn't start a fight but am very aware of the damage he could do if he retaliated.
I do try to walk him where there are few other dogs to encounter but the problem with this is that you are then unsocialising your dog and they don't know how to behave around others. In conclusion I rotate and try to mix with other dogs as much as possible in a controlled environment (for socialisation) and walk when not many others are around but also pull in some long countryside walks where he can run free without fear of attack from a smaller dog.
Have fun
Sue & Rory the Leonberger
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I have exactly the same problem with my wolfhounds and deerhounds, though its usually only with little dogs. Fortunately the big ones just look puzzled :-).

Just been discussing this very thing with a friend that I meet whilst out dog walking, I had my young 5 mth rotty and smaller shiba with me and she had her sons rotty and her own GSD cross, the only dogs we had problems with all the way round the walk were small ones! my friend says it's like the 'small bloke with attitude' problem, the smaller they are the more they feel they have to prove themselves!!!!! :D