A flying pegasus can fit through a 1-block gap

src/main/java/com/minelittlepony/unicopia/Unicopia.java

@@ -156,7 +156,6 @@ public class Unicopia {
     public static void onBlockHarvested(BlockEvent.HarvestDropsEvent event) {
         if (event.getState().getBlock() == Blocks.STONE) {
             int fortuneFactor = 1 + event.getFortuneLevel() * 15;
-            System.out.println(event.getFortuneLevel());
 
             if (event.getWorld().rand.nextInt(500 / fortuneFactor) == 0) {
                 for (int i = 0; i < 1 + event.getFortuneLevel(); i++) {

src/main/java/com/minelittlepony/unicopia/mixin/MixinEntity.java

@@ -0,0 +1,34 @@
+package com.minelittlepony.unicopia.mixin;
+
+import net.minecraft.entity.Entity;
+import net.minecraft.entity.MoverType;
+import net.minecraft.util.math.AxisAlignedBB;
+
+// pseudo code for things forge can't do.
+// @Mixin(Entity.class)
+public interface MixinEntity {
+
+    // @Accessor("setSize")
+    static void setSize(Entity self, float width, float height) {
+        if (self.width != width || self.height != height) {
+            float f = self.width;
+            self.width = width;
+            self.height = height;
+
+            if (self.width < f) {
+                double d0 = width / 2;
+                self.setEntityBoundingBox(new AxisAlignedBB(
+                        self.posX - d0, self.posY, self.posZ - d0,
+                        self.posX + d0, self.posY + self.height, self.posZ + d0));
+                return;
+            }
+
+            AxisAlignedBB axisalignedbb = self.getEntityBoundingBox();
+            self.setEntityBoundingBox(new AxisAlignedBB(axisalignedbb.minX, axisalignedbb.minY, axisalignedbb.minZ, axisalignedbb.minX + (double)self.width, axisalignedbb.minY + (double)self.height, axisalignedbb.minZ + (double)self.width));
+
+            if (self.width > f/* && !self.firstUpdate*/ && !self.world.isRemote) {
+                self.move(MoverType.SELF, (double)(f - self.width), 0, (double)(f - self.width));
+            }
+        }
+    }
+}

src/main/java/com/minelittlepony/unicopia/player/PlayerGravityDelegate.java

@@ -2,6 +2,7 @@ package com.minelittlepony.unicopia.player;
 
 import com.minelittlepony.unicopia.InbtSerialisable;
 import com.minelittlepony.unicopia.USounds;
+import com.minelittlepony.unicopia.mixin.MixinEntity;
 
 import net.minecraft.block.Block;
 import net.minecraft.block.SoundType;
@@ -35,6 +36,7 @@ class PlayerGravityDelegate implements IUpdatable<EntityPlayer>, IGravity, InbtS
 
     @Override
     public void onUpdate(EntityPlayer entity) {
+
         entity.capabilities.allowFlying = entity.capabilities.isCreativeMode || player.getPlayerSpecies().canFly();
 
         if (!entity.capabilities.isCreativeMode) {
@@ -43,6 +45,13 @@ class PlayerGravityDelegate implements IUpdatable<EntityPlayer>, IGravity, InbtS
 
         isFlying = entity.capabilities.isFlying && !entity.capabilities.isCreativeMode;
 
+        if (isFlying) {
+            MixinEntity.setSize(entity, entity.width, 0.5F);
+            entity.eyeHeight = entity.height / 2;
+        } else {
+            entity.eyeHeight = entity.getDefaultEyeHeight();
+        }
+
         if (!entity.capabilities.isCreativeMode && !entity.isElytraFlying()) {
             if (isFlying && !entity.isRiding()) {