Any injection that causes inflammation can lead to caner. In general, this is less likely with Depo Medrol - so this is good.
I would however talk to your vet about alternatives to Depo Medrol. With each injection of depo-medrol, cats run the risk of temporary or permanent diabetes. There are other risks as well.
There are many other safer options if the cat needs long term steroids. One a one time basis, depo medrol is fine but if it is given more than 2-3 times (in a life time), then other options should be considered.
For asthma, inhalant steroids work well and are fairly easy to administer. For IBD, etc, oral steroids like prednisolone in a chicken flavored liquid can easily and cheaply be given on top of the food.
In general injection lumps should get large after a few days and then slowly resolve over a couple of weeks. If after the initial presentation of the lump, the lump enlarges, becomes firm or warm - then a re-evaluation by your veterinarian should be done at once.
I hope this helps.