well i just measured an old 1700 cell i have and with solder on the ends and heatshrink wrapped around it it measured about 23x43 so there's the difference... also the c cells you are looking at might not be nicd or nimh, they could be something else
oh and by the way 3000mah at 30a will last for 6 minutes (0.1 hours)
3000mah(or 3ah) at 30A means 0.1hours