Firewood Resources

In RTS games, wood is a common resource. However, when workers gather it, it's always shown as tiny bundles of wood.

In real life, this would only be useful as firewood, not for building structures. The main reason for this trope is because it would be much harder to animate several workers hauling logs than it is to animate single peons working by themselves. See Space Compression, Units Not to Scale.

Alternatively, may be because you need miniature planks to make all those tiny buildings.

Examples of Firewood Resources include:

Age of Empires
Dwarf Fortress for the default, unmodified game, has logs go directly into carpenter's workshops and be used directly in constructions - even if the construction is a smooth flat road
Populous
Units in Warcraft go through the trouble of chopping down trees and then forming them into nice little logs for you before actually bringing them back to the lumber mill.
- In the first two games, they're obviously carrying long planks.
- World of Warcraft pays homage to this by having many quests that involve collecting those neatly stacked piles of lumber, ostensibly for construction purposes, although some do actually make them look like cut boards, not firewood. Justified to an extent by the Goblins who, not to be outdone in the efficiency department, use machines called Shredders that can automate the chopping and sawing of wood until they spit out a nice collection of perfectly formed boards. Until they break down and explode, or go berserk and start killing people.
Rise of Nations - coal cuts down on wood costs, implying they are used for fuel. Wood is still used for construction in the game though.
Heroes of Might and Magic - maybe, it could be interpreted as logs since scale is questionable
- However, the back story behind a certain artifact called endless bundle of lumber, which provides the owner with one extra wood each day, says it's a small bundle which the person who found it, first mistakes for firewood.
Cultures series
Age of Wonders - Small wood bundles were terrain items that gave your closest city an instant structure building bonus
Pharaoh - although they turn into planks when sitting in your storage yards.
"Virtual Villagers" and its sequels
RuneScape - While it's not a RTS, all logs obtained via the Woodcutting skill are a bundle of three logs.

Aversions

Black and White - the wood become piles of planks, and they look like long logs while being carried.
The Settlers - always shown as planks
- Actually, first the woodcutter visibly chops down a tree, strips it of the branches, and carries the log on his shoulder to his house. This is carried by other settlers all the way to the Carpenter's house, where he cuts the log into planks, which are then carried off to wherever they are needed. Half the uniqueness of the game comes from actually animating all those things, where other games are too lazy or too abstract.
  - Much of the other half comes from the gameplay implementation of those animations. Nifty animations aside, the fact that your little animated settlers actually carry resources from A to B forces you to organize a transport network that can handle the flow of resources without getting into traffic jams. This game averts Easy Logistics hard, to the point where logistics is the main source of challenge in the game.
  - This also produces the weirdness that one tree turns into one log turns into one unit of planks; the same amount of planks can be used to build either the frame of a small house, a rowboat, or the handle of a spade.
Stronghold - the wood becomes planks that are at least as tall as the worker, and sounds a great deal like the settlers above.
The Caesar games, where you play the governor of a Roman province/city has Timber Yards that produce planks. The only use for these planks is making furniture or exporting them. Which makes you wonder just what the heck the city is being built out of.
Minecraft may not be a RTS but it does avoid this trope. Breaking a tree gives you "log blocks" which have no other real purpose than to be crafted into four "plank blocks".
- Later on, the ability to smelt wood into charcoal was added.
Tropico Lumberjacks always carry single log, the tree they just cut down. They are still taken futher by teamsters on wheelbarrow, and need to be taken to a lumber mill to be turned from raw logs into useful planks of wood. Lumber can be exported, but it's further useful when taken to a furniture factory...